Afraid I haven't had the pleasure of doing Deepground yet, if it was an exploit/glitch however they may have fixed it in a patch recently.

My suggestion is if at all possible look for trash mobs around the NPC and build up some TP before the first wave is summoned, then rush into the fray and Steel Cyclone everything or have a DRG Invigorate before the first wave and open with Leg Sweep to at least give you all a few seconds with it's stun effect. It might not grant you hate for long but it should give you at least the initial hate you need to save your WHM's. Just make sure to save some TP for future and oncoming waves as well.