Persistent Hydra

If damage would be dealt to Persistent Hydra and the amount of damage dealt would be unequal to it's toughness, instead prevent that damage and put X +1/+1 counters on Persistant Hydra, where X is its power.
Persistant Hydra can only be destroyed by damage.
You have to get it in just the right way to make any difference. One wrong move and it becomes exponentially stronger.