The strength of this card is debatable.
Against certain creatures, it's perfect:
Otyugh 1/3 or 1/6 (after devouring 1 creature) - useless otyugh unless his hp is buffed/or attacks again
Against other creatures, it's a bit pointless:
Cockatrice 4/4, Horned Frog 3/3, pretty much anything with equal offense to defense.
Against a number of creatures with adrenaline it is a penalty to yourself:
Armaggio 1/25 with adrenaline attacks once for 1, attacks again for 25, returns to a 1/25 creature.
Another odd example
A spark would attack once then become a 0/3 creature.
Think this has been said before, but I think if a creature with antimatter was to attack (ie, negative offense) it should still be affected - ie, dying. So the heal would go through and then the creature would die. This at least gives Entropy a way of creature control other than the very rare case of Mutation causing an instant kill. My thought behind this is, you play this card, wait for your opponents colossal dragon to attack once - antimatter it, gain mega hp back and he loses his dragon... Sure its very case specific, but how often do you think people would 1) use this shield or 2) use antimatters?