Guan Yu Paladin
Snow White Archbishop
Put a Double Strike Rune on Rhea
Rhea builds ATK
Guan Yu and Snow White build DEF
Items you might want to bring:
Major Healing Potion
Major ATK Root
For every turn that your Rhea is close enough to Snow White, you have to use Void Guard and Defense Barrier on her. This is because there is currently a bug where if Dark Elemental charms your Rhea and your Rhea does enough damage to Snow White, the damage will be applied to Rhea instead, causing her to commit suicide.
The aim is to defeat all enemies and have Paladin's aura up before you enter 2/2.
Once you have killed all the melee enemies, remember to move the Heroes far away from one another so Rhea cannot hit Snow White, preferably with Rhea all the way at the back.
Cast Snow White's spell before leaving the stage. You can also cast Rhea's spell before leaving too. Just remember to use a Cleansing Potion on her at the start of 2/2 if you decide to do so.
Cast Paladin's spell on Rhea. Watch her take out everyone. You can choose to use Major ATK Root for maximum effect.
After all the minions are killed, you just have to use your items to ensure just one Hero always has more than 50% HP. Just let the other two Heroes and your ally perish.