In order to finish the DLC mission you must open the ship docks door and let Lexine escape. The biggest problem is, you must kill yourself to open them because you lost your leg.
As you lay there bleeding out you need to shoot 4 fuses on the bay doors to get them to open. I suggest shooting all but one when you are first down.
You will then have around 1 minute to kill sixteen really easy to kill necromorphs. I was attacked by Slashers and the dudes with expolding arms. The pulse rifle made extremely short work of all of them.
*At one point I grapped a Slasher's arm and shoot it though another oncoming one. they are weak enough that they fly a great distance. I was able to get "Going for Distance" achievement doing this.*
As soon as the achievement pops, shoot the last fuse and open the door. don't wait, because you will be close to bleeding out and having to restart from the last checkpoint (it's right before you loose your leg, so not a huge deal if you do bleed out)