Berghia Nudibranches work for some. They didn't work for me though.
http://www.saltyunderground.com/
As suggested, a matted filefish might also work.
Instead of 2 peppermint shrimp, you could try adding 10 more for a dozen or so total.
Whatever approach you take, it's a numbers game. You need more predators than prey, so to speak.
I solved my problem by throwing out all of my live rock (150 pounds) and starting with new base rock. So far, the strategy seems to have worked. Honestly, the financial loss was worth the end result of being aptasia free.
- Brad