Play on co-op, go to the gravity lift, kill all aliens, when you are done don't go onto the gravity lift. Go to the upper left side to the single tree that is between a big boulder and a little boulder. Have your partner jump on the big boulder then jump on your partners head and on to the tree. Get high as you can in the tree, then keep killing your partner & turning until they re-spawn on top of your head. Then jump on top of the ledge and kill your partner to pop them up next to you.