This is a story related achievement and cannot be missed.
After defeating general Kyong you will be on your own and have to find a way out of the structure. If you first set step into the alien structure it´s not that hard but afterwards you will find yourself again in zero gravity. Try to follow the flashlight on the walls and you will find your way. Sometimes you will need to shoot your way through kind of grids and also in the huge room where you first encounter those aliens punch the energie sources so you can fly through the pipes. Later on where you have those invisible walls just wait a bit and try to fly through whenever its possible. Pretty close to the end you will have to defend yourself against a huge amount of aliens while the energie for the doors is loading. Try to stay in armor mode so they cannot kill you with their head stomps and if posible grab and throw them until the loading sequnce is finished.