This can be done on any difficulty, obviously beginner will be the easiest option.
This can be done solo:
Load up escape, go into map sharing and search for the map called Reinforcement Rush.
This map is pretty straight forward, and you can beat it in about 2-3 minutes if you follow my guide below.
Click on host game with map, Choose one of the three characters:
Mac, Lahni, or Keegan.
I choose to go in order so I started with Mac, finished the map five times, and if your achievement tracker is working, after finishing the map five times with one character it should be at 33%, second character 66% and so on.
When you have chosen a character press
to start the game.
For some reason when you die and chose to restart the act, the only enemies that will spawn are the ones in front of the doors you have to go through, so:
When you spawn don’t move and when the timer hits zero, the explosion will kill you, and choose to restart act after this.
After spawning again:
Go through the first door you have to open and go to the right supply room and pick up the boom shot.
Open the next door and If you go straight you will see a group of 3 enemies use the boom shot and finish them off any way you want. Go through the door and repeat the last step, killing another group of 3 enemies and going through the door. Keep going straight and you will see the end of the level.