This achievement is surprisingly luck based based on (where you join the game, what levels you are given, who is in your lobby, What chaos you are given, who your teammates are, how many people follow through to the end) and that is all before you even get to the 'defend the vats' part. If you get to the end part with lower then 1625 chaos then you can't get the achievement at all and you have to do the mini missions all again (You will be told how much chaos you have just before the end defend mission). When you get there however here are some useful tips:
Unlike the other three chaos missions in my experience it includes all of the special OD
It seems that only the normal OD can wear down the vats so focus on them
Special OD break traps so replace them when they are destroyed
For how I did it, I essentially just spammed every explosive weapon and pyro trap I had into the OD spawning locations as you can shoot through the invisible walls on the sides. This works until wave 3 where there is so many special OD I just alternated between being dead and spamming weapons until the game ended. For the special OD in the air I just ignored them and I managed to complete it.
For what I would suggest, Use the freeze gun as it gives you time to breathe and use explosive weapons such as the teddy bear launcher and the hair spray launcher as they have large blast radius' taking out many OD and they damage the Special OD at the same time.