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!). Don't kill him to many times either or he will kill you (one shot your dead). BEWARE the corners if Johnson gets stuck in them grenade him out.He can get knocked out and get annoying because since he doesnt move by himself he can get stuck blown and get whoknows, where so keep an eye on him.
When you visit the ops center again you should take (whack) Johnson with you to meet the Johnson clone gaurding the bomb. Don't be Greedy you cant whack the one guarding the bomb around (well at least i dont think you can) without him walking back to his post.
To sum it all up LEGENDARY EASY WITH A SLAVE NAMED SERGEANT JOHNSON. WHACK HIM AROUND AND HE WILL DO ALL THE WORK!