When an enemy's health gauge is purple, it means a biotic barrier is active.
Either use warp yourself or order a squad mate to use warp on an enemy's barrier to count one towards this achievement. The warp must completely remove the barrier, so you may need to soften up the barrier with gunfire or use two warps at lower levels.
If a squad mate uses warp on an enemy without you ordering them to do so, it won't count towards this achievement.
Progress on this achievement will show in the corner of the screen when you're successful; you can also check progress using Shepard's terminal in his/her quarters aboard the Normandy.