There is a huge amount of great advice here, so I'll keep this brief. For me, as a white mage, I ensure regen is up indefinitely, making sure to cast it at best effect. When death sentence is being cast, the tank is generally on full hp. This makes it easy for me to ensure a very quick recovery (with a little assistance from the scholar in my group, of course). When death sentence is about 60% I start casting Cure II (With divine seal if possible) and it should hit just as death sentence does, giving the full effect, and ensuring the tank has plenty of hp for any subsequent attacks that may be quickly executed afterwards.

If you have down periods where healing is low, having stoneskin up can be infinitely helpful. Assuming the tank has 7000 hp, that's pretty much a "pre cure" of 1260.