This trick is relatively easy, but requires patience. First you start off in the level, but with no guns. You have 2 options get the Black Eye skull (on pipe above) or you can go downstairs and get a gun. Now go up to the ops center and go to Johnson. Now this is the time consuming part. Whack Johnson around the whole level. He will do all the fighting just give him a weapon (battle rifle works nice). Do this by killing him and drop a gun on top of him. If you sont give him a weapon when he gets shot he makes an atemmpt to run over and try to punch the covenent forces (hilarious!).

This trick is very easy to do. Go on the last level and keep a rocket launcher. Next play through the level until you get right before the door to Guilty Spark 343 (outside the door of the cutscene where Sergeant johnson dies). Then if you already haven't let the Sergeant catch up and take out your rocket launcher. Shoot him (make sure he dies) and go into the cutscene before he gets back up. Kill 343 and when you exit the room Johnson will still be with you. Now prevent the Arbitor and the Sergeant from destroying the infection forms. If done correctly an infection form can enter Johnson.