Since there is no confirmation on what the achievements will be for the game, aside from the time being, let's create our own!
Here's some of mine:
"KEEP IT CLEAN - 10 GS: destroy all environmental objects within the city."
"Terminator - 50 GS: Never use a health pack"
"Ray Charles? - 50 GS: Never use the low-light mode, VISR"
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Bad Idea - 20 GS: Kill an ally with a rocket launcher. (Flashback to Halo 2 New Mombasa. Brr.....never give you allies a weapon that can kill you)