More than a Handful - Kill 125 enemies in multiplayer Matchmaking - 2500cR
Nice and simple. Only other MP challenge today involves close combat, so grab your Swords & Hammers for some Grifball explosions.
That Pink Mist - Kill 100 enemies with a Supercombine from the Needler or Needle Rifle in Firefight Matchmaking - 2500cR
Not in one game, so that's nice. I'd suggest some Arcadefight, you get a Needler/Needle rifle in one of the loadouts and it has unlimited ammo. Very nice and simple. just unleash fun.
Just a Shower - Complete any Campaign mission on Normal or harder with Thunderstorm on - 4000cR
You can die and it's not even Heroic difficulty. I still say Nightfall for speed purposes but any level will do.
Up Close and Personal -Kill 5 enemies with close quarters combat in a multiplayer Matchmaking game - 2343cR