Evacuation is a standalone HL² adventure primarily created by Simon Stakhowsky. It consists of a single episode spanning five maps that puts the player in the role of a rebel soldier traversing White Forest in search of a way of rejoining his comrades. The mod takes between 90 to 120 minutes to complete and offers a total 15 achievements. This guide lists the achievements in order of appearance and provides reference shots as well as tips to get them all in a single run.
Considering you are a regular shooter player, Medium difficulty is recommended for a balanced first run. It is also advisable to make manual saves at the start of each map in case you need to return to them to get a missed achievement.
It may also be better to set RMB to 'Sighting' (ADS) instead of 'Secondary Fire' since firing from the hip is pretty inaccurate and secondary fire is only needed to fire both barrels of the shotgun and launch contact grenades from ARs ('X' may be an ideal choice for that in a WASD setup).
As a side note, a few additional variables directly related to immersion can be changed via the menu option titled 'Mode Settings'. Personally, I opted to set 'Weapon Placement' to -Medium- (feels better than close with the default FOV value of 90), turned 'Swinging with a Gun' -Off- (since it causes a disorienting effect while changing weapons) and kept 'Swaying', as well as 'Camera Shake' -On- (as they seem to provide MMod-like subtle immersion).

These should pop up at key points during a first run. The points being;
as you wake up at the crash site,
once you come across the raider camp,
after you take down the hunter-chopper at the church,
and finally, as you take the train out of White Forest.

In case the following are missed, the related maps are mentioned in italics.
evacuation_1 - Upon starting a new game, you find yourself in control of the mounted rocket launcher on a rebel helicopter flying over the White Forest. A total of five striders are wreaking havoc in the area: one in the distance (seen in the first shot), three around the tower (as seen in the second shot) and a final one right after those which is scripted to take your ride down. Be quick on the trigger and destroy the three around the tower.
evacuation_2 - Activating the tower on the first rebel camp you come across also lowers down the walkway to the next area. Make a quicksave and simply drop down to the lavine below as you go over the said walkway.
evacuation_2 - Following from the previous achievement, you hear someone yell 'please, no' just as you step over the broken plank near the end of the walkway. As soon as you hear that, sprint and take down the shotgun-toting raider behind the fence who is about to kill the rebel medic in front of him. This may take a few tries as you have only about three seconds to kill the said raider after you hear the medic cry for his life.
Note that the achievement gets unlocked only after going through the camp and the medic comes out from his hiding spot with a shotgun in hand to help you out.
evacuation_3 - Upon reaching the saw mill, you are first tasked to restore electricity to the area. The only available path at this point sees you descending down to the nearby ravine. Once down there, make a sweep from the waterfall on the left to the hill on the right and kill all the zombies and headcrabs muddling around.
evacuation_3 - After stepping out of the ravine, you follow a zombie infested path through the forest that ends up at a tent where three bandits dwell. Once you get rid of them, pick up the assault rifle that resides on the ground next to the power cable you need to take to the saw mill.
evacuation_3 - Once you insert the power cable to its intented outlets and pull down the nearby lever, an antlion guard appears alongside an endless swarm of antlion soldiers. Simply kill it to unlock the related achievement. Still, you are not required to take down the guard to progress, since the gate that leads to the next area opens up exactly two minutes after its appearance, as told over via a radio message.
evacuation_3 to the end of evacuation_5 - After killing the guardian, get inside the building with a water tank on its side and look for a garden gnome on the shelves. Shoots at it a few times to dislodge it from its place and take it with you. In typical Valve fashion, you need to carry it all to way to the end to unlock the related achievement.
Thankfully, it is not that hard - two places of note though:
> evacuation_4: Once you come to the crossroads with a tower in the middle, leave the gnome by the truck or up in the tower since you come back here later to go through the steel gate in the middle after taking the path to the right signified by a lambda symbol:
> evacuation_5: Upon reaching the residential area where you need to make your way through the buildings while being hunted down by the sniper in the church ahead, the better option would be to rush to the dropship container right ahead and throw the gnome to the other side of the fencing before moving on the usual path. If that proves hard, either go through the buildings with the gnome at hand or leave it by the entrance and return to it after clearing out your path till the other side of the fencing in question (if need be, a nearby chair comes in handy for going back through a certain hole in the wall):

As before; in case the following are missed, the related maps are mentioned in italics.
evacuation_4 - After opening the wooden gate to the right of the tower and dealing with the two fast zombies rushing in, a bird descends to the ground before you, only to get killed by the bear trap on your path. The idea is to kill it before the trap does. So, make a quicksave before passing through the gate, sprint towards the trap in question and launch an AR grenade (the easiest option) towards the bird as soon as you see it descending down.
evacuation_4 - Following from the previous achievement, the path you are on leads you below a cable car that needs to be powered up via a nearby tower before it can be used.
To divert the power, first you need to get the blue keycard from the building next to the tower and carry it to the locked shed nearby to unlock it. Get the power cell from the shed (do not miss the other unique weapon on the floor either) and insert it to its designated socket behind the said building. Doing so detaches the plug from its outlet. Make a quicksave and keep holding the live plug until you die from the current.
Once the achievement is unlocked, quickload, remove the cell from the socket, insert the plug back to its place and replace the cell (as shown in the fourth screenshot). After doing so, get back into the building and pull the middle lever to complete the sequence.
evacuation_4 - Following from the previous achievement, return to the now powered cable car. This is trickier than it has a right to be, so make a manual save before getting on it. Note that there may be other ways of handling this one other than the method outlined below: feel free to add in yours via the comments section.
The ride sees you battling it out with a gunship while under fire from the sniper hiding in the shed ahead. Stay crouched behind the rocket crate and take care of the gunship with the provided rocket launcher. Let the overwatch in the valley below to themselves but deal with the manhacks without letting go of your crouched position. Once on the other side, step out of the car to make the sniper leave his spot and kill him.
The idea is to destroy the other cable car while it is moving by hitting the mechanism at its top with your body. Switch to Easy difficulty (so that you are not killed instantly with your own rocket explosion), heal yourself fully and make a quicksave.
After that, take care of the remaining soldiers below. Turns out that killing the last one spawns *another* sniper that immediately goes for the cable car adjacent to yours as long as he does not see you in the process. Now turn your back to the leaving car, hold your launcher at around 40 degrees towards the ground and jump-fire to steer yourself towards the top of the car in question. Of course, this may need a few retries.
evacuation_4 - Once your battle through the riverbed is over, keep following the path to its very end before powering up the gate on the left to find two supply crates obscured by the waterfall.
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Still here? Here is a helpful binding for taking 'clean' screenshots then -
Feel free to comment if you require further assistance.