One Spartan Army - Kill 100 enemies in Firefight Matchmaking - 2000cR
A single game of any Scoreattack Mode will be fine.
Single Wield - Earn 100 kills with small arms weapons in multiplayer Matchmaking - 3000cR
SWAT Magnums for the one hit kills.
Roast 'em, Toast 'em - Kill 10 enemies while jet-packing in Exodus on Normal - 2400cR
RAlly Point Alpha, Grab the Rocket Launcher from the ODST, Grab a Jetpack and hit those Grunts WHILE HOLDING DOWN THE USE EQUIPMENT BUTTON (Won't get the medal otherwise). Revert to last saved, do it again.
Iron Will - Complete a Campaign mission alone on Heroic or harder with the Iron Skull on - 2500cR
Nightfall, Leg it.