Easily obtainable on the first level, "The Gates of Dragonfall".
Set up two Lightning Strike Auras at each enemy spawn. If desired, put a Flame Aura after each set to catch stragglers that are missed.
After Wave 2, remove Flame Auras, if added, grab new gems, and add more LSA's.
Near the end of the second wave (there's no build phase for Wave 3), grab gems and add more LSA's, add two Flame Auras for the middle and right lanes, leave left lane near Crystal open so the boss doesn't die. As long as the boss is alive, new mobs will keep spawning.
Should easily obtain the achievement this way!!
For those that like a video guide.