Simple strategy. Works to soloheal on Normal or Hard, or Elite with minor modifications and maybe one or two mana pots.
1) Turn Quicken on. Elite *only*, also turn on Empower Healing.
2) Target a high HP melee (Barb 20's are excellent choices, as are dwarven rangers).
3) Cast Mass Heal.
4) Check your own HP. If under 300, cast Heal on yourself.
5) Wait until cooldown of Mass Heal is complete.
6) Return to step 2.
Melees do not fight through blades, although the one you have targeted is last to leave. Noone dies unless they are undergeared (under 400/500/600 hp n/h/e or lacking Heavy Fort, or a WF with no healing amplification). Harry simply doesn't have the DPS to outdo your healing. And regardless of lag, your melees are getting a big heal every 6.5 seconds which keeps them alive.