In MvM, the Medic plays a strong defensive role, while also providing offensive aid with the occasional crit charge/canteen. His four main jobs are:

• Ensuring his teammates stay alive through his medigun.
  
• Shutting down projectiles from robots, especially giants, using the shield.
  
• Buffing teammates with crits/Übercharges so that they can hold their ground and deal more damage.
  
• When things go south, reviving teammates so they can get back into action more quickly.

Medic is one of those classes that feels really important in MvM, both while playing it and while having one on the team. But before I go on, let's get the elephants out of the room.

The Medic is NOT some god-tier absolutely necessary class. It is completely possible to beat missions without a Medic. In fact, in Expert missions the consensus is that Medic isn't a core class. Hopefully that takes care of all the diehard Medic fanatics who absolutely MUST have one on every team.

Next up:

The Kritzkrieg is NOT the only medigun in MvM. People consider it the default medigun of MvM, mostly because the periodic damage spike is useful, and because it simply leaves less room for error than the other guns.

However, it's not the only medigun that can be used in MvM. Believe it or not, there are actually an entire three other secondaries the Medic can use!

• The stock Medi Gun, which provides temporary invincibility with its Übercharge.
  
• The Vaccinator, which provides enhanced damage resistance and quick revives.
  
• The Quick-Fix, which provides rapid healing and immunity to knockback.

There are very few "bad" weapons in the Medic's arsenal; almost every weapon can be practical as long as it's used correctly. Unfortunately, not everyone uses their loadout correctly, so here's some basic tips for getting your weapons together.

A note on primaries: As a Medic, you shouldn't have to pull out your primary weapon too often, unless applying Mad Milk syringes, so this choice is really a matter of personal preference.

Now that you've got your loadout set to go, let's talk upgrades.

In any mission, this should be the very first upgrade you buy. The shield is one of the Medic's major new mechanics from the Two Cities update, and it should be utilized to its full potential. It also functions as a weapon, dealing some decent damage too, so don't be afraid to run up to robots and vaporize them yourself. Only one point in the shield makes it smaller and less extensive horizontally. Both points in the upgrade means the shield covers your entire field of view.

This should be the second upgrade you get as a Medic. Faster heal rate is obviously desirable for many reasons, but it also means you revive faster. This is a must have, especially while using the Vaccinator, which has a slower overheal build rate by default.

This upgrade is pretty useful for keeping teammates topped up and fighting at full potential. At max upgrade level, this upgrade keeps your teammates overhealed for much longer, with a much, much slower overheal decay rate, meaning you don't have to top them up so much. It should be noted that this upgrade is substantially more useful while using the stock Medi Gun or Kritzkrieg, as they build overheal quickly compared to the Vaccinator, and they grant more overheal than the Quick-Fix.

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Upgrades branch out from this point on. Make calls depending on the upcoming wave composition, and upgrade accordingly. In no particular order:

This resistance is quite possibly the most important resistance to have. The full upgrade only costs $450, and easily means the difference between getting killed by one stray crocket and tanking a direct hit. If the upcoming wave has masses of crits/minicrits, get this.

When you have money to spare, this upgrade is really nice to have. It extends your Übercharge from 8 seconds to 14 seconds at max upgrade, which means you get Über for your charge. This is ideal for every medigun, especially the stock Medi Gun and the Kritzkrieg.

This upgrade allows you to apply Mad Milk to any robots hit by your syringes, with 1 seconds of milk for every syringe hit, up to a maximum of 4 seconds. It's helpful if there are multiple giants and you have no Scout/he rarely milks. However, keep in mind any time you're applying Mad Milk syringes is time you're not healing your team. Note: This upgrade does not work for the Crusader's Crossbow.

This upgrade can be really great to have or it can be a small convenience, depending on which gun you're using. For the Quick-Fix or Vaccinator, Crit canteens help make up the lost damage, while for the Kritzkrieg and Medi Gun, Über and Crit canteens, respectively, help round off the mediguns and give nice buffs in a pinch.

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Notice how Übercharge Rate isn't on the list. Don't bother buying it, it's a rubbish upgrade unless you're absolutely rich in the last wave. What Übercharge Rate tries to do, proper use of an Übersaw does much better.

The main Medic strategy is as simple as healing your teammates, shielding damage, and reviving when necessary. Of course, in practice it becomes much more nuanced than that. Here are a few tips to keep in mind whenever playing Medic in MvM.

• HEAL EVERYONE. This one gets all the emphasis because it's literally the reason Medic even exists. This means stop only healing your pocket Heavy or Soldier and learn to heal everyone else on the team. See the Scout retreating from fire? Give him a quick buff with the medigun to get him back on his feet. Hear a Sentry Buster's about to come? Pay special attention to the Engi and make sure he doesn't die while hauling his sentry, especially if he has the Rescue Ranger's minicrit debuff. Hell, even if the Sniper's calling for you, go out of your way to spare him a crossbow shot or two.
  
  Something else to add: By healing more than one or two people on the team, you build your shield much faster. If you keep pocketing the same person, you might get a shield up every minute or two. By healing everyone on your team and topping up everyone's overheal, you boost your shield charge rate substantially and can pop out a shield every 20 seconds or so.
  
  
• Know your limits. The shield is only useful for so much; it does actually take some time to kill robots instead of vaporizing them instantly, so if you run right through a crowd of Soldiers expecting them all to die, you're going to explode. If you see someone dead on the other side of the map, realize that they might respawn before you even make it to them, wasting your time in the process.
  
  In the same vein, if you see someone die, but there's a horde of robots between you and their Reanimator, don't revive them unless you absolutely know you can get out safely. A dead Soldier is better than a dead Soldier and a dead Medic.
  
  
• Use the shield correctly. The shield is both a weapon and a defence, and if you're only using it one way you're using it wrong. It's entirely possible to shield your teammates from enemy fire while at the same time being in the sweet spot where your shield can damage them. This applies to giants too; in practice, there's a very fine line where you can damage a giant and block their fire at the same time. It takes practice to master, but a general rule is that if the front half of their weapon is past your shield, you're too close.
  
  While shielding, you want to stay as close to the bots as possible without putting yourself in danger. By staying close to the bots, you make it easier for your shield to damage them and you allow your teammates to safely get closer to the bots and benefit from damage rampup.
  
  
• Revive people. This is much easier with the Vaccinator usually, but revive everyone you can. If the Engi just died and a Sentry Buster's on the way, revive him so he can make sure his sentry doesn't go down. If your Demoman is dead and there are Medic bots on the way, prioritize reviving him so they can be taken down.
  
  Really, the order in which you revive comes down to the situation and common sense. The only class who should be revived first in every case is another Medic (assuming you have two Medics for whatever reason); the second Medic can make reviving everyone else faster.
  
  Keep in mind that revives take longer or shorter depending on the revivee's class and how often he's been dying recently. A Heavy will take the longest to revive, while Scouts, Engineers, and Spies revive fairly quickly.
  
  
• Stay conscious of the ♥♥♥♥♥♥♥ Sentry Busters. I've died countless times to careless Engis who detonate their Sentry Busters smack in the middle of our wall. When you know a Sentry Buster is coming along, make sure to stay clear of it, and stay wary of it even when it's out of sight until it's exploded for sure.
  
  Furthermore, if you can see that a Sentry Buster will almost definitely kill a teammate, get ready to revive him as soon as possible. If possible, you can try the instant revive glitch by aiming just over their head while healing them before they die.
  
  
• By extension, stay aware of everything.
  
  • Glance at the bomb compass every once in a while. If you're on the front lines, but you see the compass is pointing behind you and the icon shows the bomb's being carried, you should go check on the bomb and make sure a Scout isn't making a stealth bomb capture (it happens all the damn time.)
    
  • Consider the possibility of Spies all the time, even if the HUD icon hasn't showed up yet. Spies are only announced when one is spotted or killed, so it's entirely possible for you to get stabbed before they are announced to you. The only time you're safe from them is right after it's announced that they have all been killed, or on waves with no Spies at all.
    
  • Pay attention to where Snipers are on the map. If you see a Sniper across the map, call him out to your team and try to stay out of his sightline until he's dead. On the flipside, nearby Sniper bots can make for excellent Übersaw targets, as it takes them 3-4 seconds to actually pull out their melee.
  
  
• Use your tools to their fullest. If you have an Übersaw, use it frequently, as the extra Über can help dramatically while using any medigun. However, be cautious to not get too aggressive and overextend, as you're an easily killed target. If you're not sure you can Übersaw and get out alive, you can use an Über canteen to protect yourself while farming all 4 hits off a giant.
  
  
• Use your Übercharges frequently and appropriately.
  
  • If using the Kritzkrieg, pop the crits on a teammate as often as possibly while maximizing effectiveness by ensuring there are multiple targets to hit with the crits. It's better to wait a few seconds for giants to spawn than to use the Kritz charge on a group of normal Pyros.
    
  • If using the Vaccinator, you should always pop an Über while reviving someone. When doing this, remain conscious of the damage types surrounding you, and switch resistances accordingly before you Über to ensure you're not killed while reviving your teammate. Vaccinator resistances can't be changed in the middle of an Übercharge, so make sure you have the right one selected. Vaccinator Übers aren't just for reviving; they can serve as a quick defensive buff in a pinch.
    
  • If using the Quick-Fix, pop the Über whenever you need a shield immediately or if you or your pocket are low on health. Remember the Quick-Fix Über overheals you as well as your patient, so you can use it as a last-ditch effort to keep yourself alive as well.
    
  • If using the stock Medi Gun, use your Übercharge when you need to get up in a giant's face and deal some point-blank Pyro/Heavy damage. Most giants will go down from concentrated fire by the time the Übercharge is over. The stock Übercharge is also useful against instakills like Spies and Sentry Busters; if you know for certain that your Engineer or Heavy will get killed by an imminent Sentry Buster explosion, Über them to save their hides. They'll both love you for it.
  
  
• Above all, keep yourself alive. You're not doing anyone any favors by kamikaze-reviving the Scout. By making sure you stay alive, you make sure everyone else stays alive. If there are hordes of projectile classes coming and shielding isn't an option, take cover and heal your teammates from around a corner until the shield is replenished. Being careless with the shield can lead to you being flanked and killed as well; if you're not confident in your ability to use the shield correctly, you can at least use it for defense and avoid getting too close to bots.

There are other, more situational things you need to keep in mind while playing Medic.

• Pay special attention to the Engi when Sentry Busters are announced. If he dies during this period, make sure to revive him as his sentry or dispenser could go down.
  
  
• If you have a Spy on your team, make sure to top off his overheal whenever possible. This allows him to make more stabs and increases his survivability.
  
  
• If you know the Scout has a pretty high overheal from cash and you have some level of Overheal Expert, heal him for a second or two and it will lower his overheal decay rate from normal to whatever your Overheal Expert upgrade is at.
  
  
• If you buy Mad Milk Syringes, make sure to use it on giants especially. Mad Milk is the most helpful when applies to giants, though keep in mind the syringes won't slow down robots like normal upgraded Mad Milk. Additionally, it takes 4 needle hits to apply the maximum Mad Milk duration to a robot, which is 4 seconds.
  
  
• If using Canteen Sharing with crit canteens on a tank, first check if your patient has his own crit canteens. If he does, look for another Soldier, Demo, or Pyro to crit. If there are no other targets, try to coordinate with him so that your canteens get used first; canteens with Canteen Sharing are cheaper, and this way it's more cost-effective.
  
  
• If you spot a Spy bot, don't be afraid to pop your shield to get rid of him. The shield is a great way to get rid of Spies in general, since they vaporize as quickly as Scouts do. If you don't have a shield ready and are good with your saw, you can always melee a lone Spy bot with one or two hits, however be careful that you don't get stabbed in the process (which is a lot more difficult than it sounds.)
  
  
• Know your maps. While playing on a map, learn where the health packs are and where escape routes are in case you need to retreat. There's almost always more than one way to get to a given point; remember every way to get somewhere just in case one path is full of robots.
  
  
• Even if you're not using the Kritzkrieg, you can always switch to it quickly during setup time to give the Demoman his coveted crit stickies. After the stickies are laid, you can switch back to whatever you were using.
  
  
• Don't be afraid to bust out the Übersaw and try to get free hits off normal robots. If it's a robot you know probably won't kill you, like a pair of normal Scouts, run in and try to get some hits on them as they drop. Just make sure you don't get killed in the process.
  
  
• It's entirely feasible to switch mediguns mid-wave. If you're using the Quick-Fix and a tank's taking too long to take down, don't be afraid to switch to the Kritzkrieg quickly to give the extra damage needed.
  
  
• If using Canteen Sharing, try to only use it at max upgrade. It increases the canteen duration from 5 all the way to 8 seconds, and with a 30% price reduction on canteens with all points in the upgrade, the money saved can be useful in the long run.
  
  
• For any medigun, using an Übercharge speeds up revives immensely. This is most apparent in the Vaccinator, which has four separate Über bars allowing for four quick revives.
  
  
• While using the Übersaw, try to put a point in movement speed after your main upgrades. It allows you to keep pace with running Sentry Busters and get a full Übercharge off them.
  
  
• In any form of TF2, MvM included, it's always better to pop your Übercharge prematurely than die and let it go to waste. If you find yourself cornered with nowhere to go, pop whatever Über you have; it increases your chances of getting out alive, and the last second or so of the Übercharge might still be useful.
  
  
• When using the Quick-Fix and a Heavy is getting ready to bodyblock a giant Scout, make sure to focus the heals on him so that he doesn't get knocked away from his post, especially against giant Force-A-Nature Scouts.
  
  
• While using the Kritzkrieg against a horde, you should use your crit charge on explosive damage classes like the Demoman and the Soldier, with the Pyro as a second option if neither are available. If up against giants, it's usually better to crit the Heavy. If you hate the people you're playing with, remember to always crit the Scout.
  
  
• When using the Vaccinator, keep in mind the passive crit negation. If you're up against a giant crit Soldier, you can switch to blast resistance and completely cancel out the critical damage without having to Über. Likewise for a giant crit Heavy.
  
  
• If you know your Engi is using the Rescue Ranger and likes to do quick saves from the Sentry Buster, try not to pop your shield around his sentry. The shield blocks the Rescue Ranger's remote pickup functionality for whatever reason, and it's possible his sentry can go down because you blocked his pickup.
  
  
• If a tank is really coming down to the wire near the end of the wave, use everything you've got on it, especially crit canteens. Your syringe gun, your shield - use any method of damaging the tank possible as a last resort.
  
  
• Remember, people will always try to find a scapegoat. If you're using a medigun other than the Kritzkrieg and your team loses for whatever reason, first consider if you're just a crap medic. If you're sure that's not the reason, remind your teammates of their own shortcomings, instead of just blaming the loss on the stock Medi Gun/Quick-Fix/Vaccinator. Just keep in mind that if everywhere you go smells like ♥♥♥♥, you might want to check your own shoes first.

The Official TF2 Wiki for item images
All the guys at the Two Cities Veterans group for being awesome teammates and giving me the experience I needed to write this guide
CrudeCuttlefish[crudecuttlefish.deviantart.com] for the Sentry Buster picture
Whoever made that one Medic shielding picture
/u/TMWHerrJon, /u/Tf2McRsWow, and Pluto for critiquing the guide and giving me some tips for it.
itsgalf for the extra tips concerning overheal decay rate and shield charge rate boosting.

Thanks for reading, and I hope this guide proves useful to many future MvM Medics!

If you have anything that you think can contribute to the guide, let me know in the comments, and if I think it's a valid point I'll add it in!