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. Later...(painful screaming noises)...Now Johnson should be fully infected. He will probuly lunge at you. Then kill him.