the difference between this and phase shield is that any means of pc can remove phase shield, but the only counter to this card i can think of is rt and lobo, so if you dont have or , and the few cards that imlement the two spells, youre sol.
As a point of order, freeze plus shockwave autokill should still be viable as well. Flooding should work too. Stun and freeze effects would counter ability and attack.
Sun dial could also stop attack (upped is usable anywhere)
The armagio trick can be countered by casting gravity pull on a different creature.
Also, what happens if the creature gets hit by mutate or improve?
If this still works and changes the creature to something smaller, it could be made devourable thusly.
Mutate still has the autokill chance.
Pandemonium can also be used since it can either make a copy (then you have a meat shield too), rewind, lobotomize, or freeze it.
All that said, I do think this may be a little OP, just not horribly so.
I also see some very interesting potential with poison, catapult... especially on voodoo dolls.