You need to get you main profile level (the one on your character card) up to level 35. You'll gain profile levels automatically while doing other achievements, so save this one for last.
You'll be using the same method as with my
solution, but focusing on Profile XP Gain gear then largest stat gear over all other gear to speed things up as much as possible.
The easiest way to earn this achievement is to sim games at 4x speed. You'll want to go to
Multiplayer->AI Battle Simulator.
Put the two characters with the most Profile XP Gain gear in the first two slots, with the third slot wearing as much Profile XP gain gear as possible just in case someone loses a round.
NOTE: Make sure you press
twice during the loading screen to speed things up 4 times normal speed.
For best results:Set up an alt XBOX Live account with 3 level 1 AI Defenders without gear that are bad match-ups against the characters you're using in your Profile XP Gain team. Friend that account. Spar with that friend for fastest, easiest games.