On the rules part, this card doesn't put counters, so it's meant as either an anti-monstrous (preventing the counters and triggering monstrous at inconvenient times) or to get your monstrous bonuses (Polis Crusher, Colossus of Akros, Stormbreath Dragon, maybe Fleecemane Lion) without overextending. It's a sorcery because no ones wants to figure out what happens if you cast it as an instant against a monstrosity activation!

Wasn't sure whether to make this monocolor, though.

I love it. I may even think something like this will come out of the cards we havent seen yet from the theros block.

I'd be surprised myself, unless there are "monstrous matter" cards.

Wants to be an enchant creature. That way, there's not memory issues.

Monstrous already invites those issues - but yeah, no need to make them worse. (Except, well, then people would expect disenchant to work, which it won't right now. Simple enough to make it do so though.)

Disenchant should work. "Enchanted creature is monstrous". Remove the enchantment and the status goes away. In fact, you could get multiple triggers off of Boomerang, which is kind of cute.