The video shows you how to keep Sgt. Johnson alive after the cutscene where he dies. You don't need to do the trick to get his spartan laser, but you do need a weapon that can kill him in 1 shot and the spartan laser is the best choice. Basically all you need to do is kill him before you enter the cutscene door. Also, he will only go to a certain point afterwards, and won't leave the building. You can push him outside by meleeing him, but it takes awhile. I managed to get him up to the little ice slope by myself, but you may be able to get him farther with more people.
I'll be posting a video on how to turn him into a flood form soon.
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Edit: Wow, 100k views, thanks for watchin all.
