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.
r: prevent the next 1 damage that would be dealt to rock hydra this turn.
rrr: put a +1/+1 counter on rock hydra. activate only during your upkeep.