The official trigger is 3 or more deaths to the same enemy, then a kill. There can be no more than 15 minutes between each death / kill to that enemy.

We tested it quite a bit in our simulator, but not in live play. The reason it wasn't working was from a program wide change we made in 10.2.0-- The update that broke existing beta 10 voice packs. This also broke this particular achievement, but 10.2.6 has that specific piece of code corrected.