If you're taking the rock out of the tank, you can just soak it in some fresh water and let the aiptasias disintegrate from osmosis. Don't do a ton of rocks all at once or you can have problems with your nitrogen cycle.
As far as peppermint shrimp, I've had them take down aiptasias that were over 2" tall. Takes them a while. They snipped off the tentacles little by little until they could get at the body without getting stung.
As far as peppermint shrimp, I've had them take down aiptasias that were over 2" tall. Takes them a while. They snipped off the tentacles little by little until they could get at the body without getting stung.