I was playing version 1.0.7.2021.01.19.152030

• <Engineer> have drone after upgrading to max skill, feel like having extra man, also have discount on buying/repairing items, recommended for beginning option.
  
• <Heavy> have infinite ammo after a few leveling up, which is pretty nice.
  
• For other characters, they are not that useful in my opinion.

Time required for first 4 level with pistol and main weapon:

• easy , 28 exp, 2 mins, 0.23 exp/sec
  
• normal, 56 exp, 2 mins, 0.47 exp/sec
  
• advance, 70 exp, 3 mins, 0.39 exp/sec
  
• expert, 84 exp, 3 mins, 0.47 exp/sec

Analysis:

• More difficult means zombies with more bug, less money drop and less sentry, result in longer play hours and lower exp earning efficiency.
  
• When you play the higher level, you will have sentry for help killing zombies, so you need to spend less effort on the earning zombie.
  
• In concluded, <Normal> is recommended

• Build 12 sentries first, you will get though 29 levels with difficulty "normal" easily. Each of the sentry would be like another good player
  
• Buy the cheapest fence always, unless you have too much money. Each fence can hold normal zombie 35 seconds, it cost 2 dollars each second.
  
• Others are good holders but the cost-performance rate is too low.
  
• The most expensive one will cost 500 dollars but hold 50 seconds for normal zombie only, so 10 dollars each seconds, 5 times worse than fences
  
• Do not buy the wall without fence, it will block the sentry detect range
  
• The tall wall will not block the laser tower attack and detection
  
• Triggering mine will not cause chain reaction to mines nearby. But the barrel will.
  
• Wood Spikes will cause damage to zombies nearby them.
  
• Building 2 walls on same spot will not stop the zombies any longer. It will only cost you extra money.

The followings are confirmed not working any more since 2021-02-21:

• In same building time slot, buying and selling same items will not let you lose money
  
• You can also sell the building that built by your teammates, and get the money that they used to buy that building. This also provides trading among teammates. You can transfer the money to engineer role players and let them buy with cheaper price (skill is researched).
  
• In same building time slot, you can buy and sell the ammo boxes without losing any money. You can use it to refill your ammo without losing any money.

• Upgrading weapon is expensive and not efficient comparing with 12 sentries. Only upgrade it or replace it when you have extra money
  
• Pistol is single shot so buying a rifle as basic weapon is better. You can buy the first weapon as it is the cheapest.
  
• "DMR"(Designated marksman rifle) is single shot weapon. It is not automatic rifle. I would not recommend buying it unless you like clicking mouse.
  
• The bullet-proof vest is extra life
  
• Dying will cause you losing the weapon but not the upgrades. Just need to buy that weapon again.

• Generate Launcher is OP, thats what everyone recommended online.
  
• The RPG have greatest damage in single shot, far attack range, but reload is too slow.
  
• Dying will cause you losing the weapon, but not the upgrades. Just need to buy that weapon again.

First 5 waves for The bridge( all level are the same)

Each zombies in different level have different rewards, range from $30-$75.
I did not go into detail.

All exp will be saved by the end of each round. So do not need to worry when you exit game by accident.

Most of them are straight forward

<Happy hour> require single game last longer than 60 mins

• If it is too difficult for you, you can kill all but keep a zombie that can not move very well. And then you can run along the field to buy enough time.
  
• There is no time limit during the zombie killing time.

You can use any characters to get through 24 to 29 level easily in solo normal mode.
There will be around 1000-1800 exp.
Pushing the whole setup upward will help you reload the sentry easier. Spreading everything among the whole field or putting them in the middle will cost you more time to get through each level.


Remarks:

• P is player location.
  Do not run near the sentry, the acid guy and bomb guy will be triggered because player is close in range.
  
  
• G is the sentry.
  If player do not come close to sentry, zombie will not attack sentry by default.
  You can reload sentry in the middle of the battle but you have to act fast. Do not let the zombies catch up on you.
  There is always a risk losing sentry when you reload it in the middle of the battle.
  Check all the sentries status between battle, Otherise fences are gone for nothing.
  
  
• O is the space.
  Keep some space for yourself in and out, reload, kill the leftover, or run away from the attack etc. You can put mines before you fight the boss in lv 20/25/30.
  Even the middle corridor is not sealed, but the zombies will only attack the fences closer to them most of the time. And Sentry will kill the rest of them.
  
  
• X is the fence.
  It can be used to slow down their moving speed for sentry. Buy the cheapest one will do, only upgrade when you have extra money.
  You can skip installing fence in the first and last rows. Give sentry some more time to kill and you will also save some repair cost.

With this setup, you can complete lv24 or lv29 easily.
For the boss in lv 25 and 30, the kind of bosses are so random and they are so challenging sometimes.
Just try your best and leave no regret.

Sample setup picture 1


Sample setup picture 2
(Only buy Tesla and the $500 wall when you have extra money. Tesla does not need to be reloaded by player, it helps to cover the sentry sometimes.)

Please help to create more guides and help the community together. ;)