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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hoo rahh!-50GS-find and discover all hidden objects in campain. do it like a grunt-10GS- throw a gernade and kill your self and 7 other covenent ODST soldier-100GS-complete the Halo 3 ODST campain on any difficulty master cheif-50GS-kill over 300 covenent with only plasma gernades in fire fight. pizza pia-20GS- kill 300 covenent with only flame gernades oh my brute!-50GS-kill a brute then take his hammer than kill his army with it.