Survive 10 consecutive waves using only custom weapons.
You'll likely get this without trying, but make sure you don't use any punches/kicks, or anything generic like a Molotov or Grenade. Your rage and the explosive barrels in the arenas will also void it. Make sure you have some kind of mod on most of your weapons as well so you don't accidentally switch to a generic weapon and void the achievement. If you play in co-op and have your friend do all the work, you'll want to make sure you hit at least one enemy each wave for it to count.