Seirawan Hydra

Deal 1 lethal damage, or 6 damage all at once. Multiple sources of simultaneous damage will work. For example, if the Hydra attacks and two Bosk Trolls defend, the Hydra will be killed.

Note that damage done one at a time, e.g. Arcane Barrage, would not kill it, since each individual 1 damage would be immediately healed.