Rock Hydra enters the battlefield with X +1/+1 counters on it.
For each 1 damage that would be dealt to Rock Hydra, if it has a +1/+1 counter on it, remove a +1/+1 counter from it and prevent that 1 damage.
: Prevent the next 1 damage that would be dealt to Rock Hydra this turn.
: Put a +1/+1 counter on Rock Hydra. Activate only during your upkeep.
