In case anyone else cares, there are 3 achievements with the Defiant map pack:
 

• Candy from a Baby (75 G) - Defiant: Capture two flags from an enemy capture zone in a matchmade Stockpile game.
  
• Crackin' Skulls (45 G) - Defiant: Kill 3 players with the Oddball in a matchmade Oddball game.
• Return to Sender (30 G) - Defiant: Successfully return two flags in a matchmade CTF game.

i am so done with halo. what kind of game doesnt let you play online unless you buy all their map packs? thats over half the game down the tubes because i am not going to give them more of my money.

I recently discovered that the Defiant achievements are attainable using custom maps as long as you're using the "official" Bungie version of the gametypes (do a search in the bungie.net forums for detailed instructions).  This sure makes getting that 150 points simple!