I have a Necro/Mesmer and find that I can handle just about ANY situation with a little attribute point adjustment. Currently I am only bringing Backfire from my Mesmer line with about 3 points into Domination. That is my main anti caster spell. It will either totally shut down a healer or kill it very quickly as Backfire is doing about 50 points of damage every time they cast a spell.
I am mainly Blood specced, but I have a few points into curses for anti-warrior hexes.
I like to start combat with Dark Pact (Sacrifice 10% maximum health and deal 10-40 shadow damage to target foe.) and follow up with Shadow Strike (Target foe takes 12-41 shadow damage. If that foe's health is above 50%, you steal 12-41 health) which totally heals me back up. Then I will spam Dark Pact a couple more times before casting Parasitic Bond (For 20 seconds, target foe suffers -1 Health regeneration. You are healed for 30-102 Health when Parasitic Bond ends.) before the mob/player dies. The Parasitic Bond hex expires when they do giving me a very nice cheap heal bringing my almost back to 100% health.
Those spells are also pretty cheap to cast power wise, and with 5 points into Soulreaping, I gain power back almost as fast as I use it up.