Really you think is overpowered? O.o
in the end it doesn't kill the creature (as fire shield) and it doesn't neutralize their ability
for example an Otyugh or a Forest Spirit with 20 hp will be in play even with the shield and with his ability still intact
(so i don't think is more overpowered than fire shield that instead can actually kill the creatures)
and, creature with high attack requires at least 5-6 rounds (non upgraded) for making them at the same attack power of the lower creatures (and they are still alive and can attack and deals damage after 5-6 round)
fire shield kills them in something like.. 3-5 rounds? (and so they deals lower or equal damage than in my case)
mhm.. your "Reverse Shield" (excuse me if i changed the name, but so i can make difference with this for now
)
give me the idea of adding to this one something like:
"...Attacking creatures have a 50% chance to get -1/-0 permanently"
(however your idea of reverse time is interesting =) and i think that i will hate it when i'll see more than 1 creature turn back in my deck >.<)