The best healer is Jigokudo. He doesn't waste heals on himself because he heals every time someone dies. So murder those clones or use clones yourself and watch his health just jump up constantly. Or use him in ranked and ninja war and watch your opponent get trolled by the unkillable healer.

When he finally dies you're probably in a bad position having not killed off any enemy DPS. If you choose to IGNORE this healer and focus on the enemy damage dealers then you're welcomed by his mystery which is a resurrection of the fallen ninja.