Fix log settings so you can see dmg taken by tank only next time you run with a war and watch if they get hit by spike dmg that should be avoided. Its probably not you so much as the tank being bad. Also as others have said be smart with your MP and take advantage of free cast C2 and Crit C3's. You can wait till war is almost dead and C3 with divine seal for an incredibly efficient cure.

I find with WAR tanks whm is put to max use, since they have such a large HP pool you can really hold off on the heavy MP cures and time them with abilities to get the most out of your MP.