The water boss is good place to practice your interrupts. The long spell, "Gahz'rilla Glacier" is the best "Ice Block" of all time. It prevents all damage and restores a lot of health. Interrupt or end up killing him twice. You can see the ice spikes below your feet before they go off, so move, move move. The pool is full of hungry fish, so try not to go swimming.
The air boss is good place to practice healing on the move since you can see those cyclones coming.
I've only healed the earth boss once. The damage spiked, requiring burst heals on 2 or 3 players at a time.