FA/EM is the best of the best for AOE damage tanks. You've already seen the pinnacle of tanker AOE with that one!!
However if you want to save your INV/SS tank, I have to open with a question: how far are you willing to go? Are willing to sell your soul? Or more to the point: are you willing to turn evil!?!?!?!!??
The best AOE damage Ancillary is Mu Mastery. Electrifying fences is technically an immobilize power, but it actually deals pretty decent DOT. So you can take Electrifying Fences, then Ball Lightning, then Mu Striker. Those first to are targeted AOE, so you just target the nearest enemy and it's the same as using a Melee AOE. The Striker will just keep hitting stuff for you, so added passive damage. You can add Static discharge if you're willing to settle for a cone AOE.