Easyest class to use for healer

[RopeDrink]

White Mage in places like Dusk Vigil during levelling roulette can be truly awful without emergency buttons, no holy, no thin air, no... nothing, really... but only if you're in a roulette with wall-to-wall tank-sprouts who probably think that it's wise to save CD's for bosses while they double pull AoE dragons and Ice Crystal packs into an inevitable CureII spamming blitzkrieg of desperation.

Pains me to say it, but if you want a healer fit for what the OP describes, then it seems to me that organizing the "Nightmare" WHM is the best bet -- a setup that makes it easy to plop Regen on people, give them easy access to Medica II, Cure II, and then whatever damage buttons you can get them to add to the four-button setup. Unless people flub mechanics, there's very few dungeons that regenerative spells won't prepare you for by themselves. Alas, it means you'll be stuck with the token regen-pre-pull overheal med-II spammer who does very little damage, but it can get the job done. Eeventually. Kinda.