Let thatbe a lesson: Never blindly copy the wording of another custom card creator. But then again the/a point of [Tribute] sets is for me to throw caution into the wind and work with a foundation of cards that I often wouldn't have created myself. Feral is a perfect example not just because ofits iffy use on static abilities, but because caring about having a single greatest power is really creating a deck that plays against itself if you have e. g. two or three feral creatures out. The mechanic is flawed in many ways.
The observation that layers are problematic here is correct. The cards needs to be changed. Triggering at combat would seem suitable, but then this card is part of a pretty tight cycle that doesn't involve only combat abilities. Granting hexproof only at the beginning of combat seems less useful.
So for now it seems like an activated ability seems the best way to go. And I actually have already a grand way to make this ability potentially cool and unique and still common. I'll update the card soon, but here is my preview solution:
Feral — {B}: ~ gains menace until end of turn. This ability costs {B} less to activate if ~'s power is the greatest power among creatures you control.
Maybe I'll be able to do something about this card being worse than
Cavern Whisperer, too, at some point. : )