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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Who needs friends? 25G - Complete 50 waves of Firefight on your own with all skulls activated. Dooms day weapon! 10G - Get 500 kills total with a Spartan laser in Campaign and fire fight.