Well if the tank is at one quarter health by the time he stops, it's him who is doing something wrong.

On huge pulls I always start with divine seal, cure two, then regen. Then cure again if necessary, before going into cleric stance> swiftcast> holy spam. Use presence of mind right after the swiftcast holy, when available. Then do at least 4 holy's, but as many as possible without leaving yourself mana starved.