This really helps if you have the mobs stuck somewhere and can cast freely at them (aka whimseyshire unicorn exploit or siegebreaker exploit).
If you are low on health from damaging them alot and have to sit and wait to regen health, you can do this to still hurt them but not take any damage:
1. Cast venom hydra near them.
2. After they shoot a pool or 2 at the mobs, recast the hydra someplace away so they stop attacking the mobs.
3. The dot from the hydra will still do its normal uber damage, BUT you wont take any of the damage reflected.
Keep in mind that the first 1 or 2 shots the hydra takes DOES reflect, it is just the dot from the acid pools afterwards that doesn't reflect. As for my build, I use Cold Blooded so i'll wait till i get to about 25% health and put up a blizzard and do that hydra trick for more dot tick damage (without hurting myself =D)