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Double Punch
While in gameplay, press the melee button (hit), then quickly throw a grenade, then quickly melee again. The entire process should take one about 1.5 seconds to do this.
Marines Hate You
On any Campaign Mode level, ( co-op or single player) just start attacking The Marines. Sooner or later, they will start to fight back, and when you target them the crosshair will be red.
Skip part of Silent Cartographer.
When you unlock the door to the cartographer place, You will fight invisible elites.
After that, go outside, look right and you will see some overshields. Get one and jump from the ledge towards the downed pelican(With rockets) you will fall, lose your overshield, but not get hurt. there will also be a warthog to hop in.