For clerics mainly, but other classes can do this too - If there are mobs on you while you are busy healing the tank you 1) go to the closest tank and stay there until they rip aggro of you, do not stop healing the tank - use stones and pots to keep yourself alive; 2) go to a DD if you know they can kill the mob before it kills them.
If you are a DD or subtank, whenever you see a cleric being attacked it is top priority to keep the cleric alive - stop attacking the boss and make sure the cleric is safe because then the whole party is safe.
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