Can I heal units that been assignes enought to kill it before it dies? I know there is one phase you're allowed to play cards and effects but Im not sure if you can heal the damage.

Karnivor said:

Damage has to be applied to a unit before it can be healed, so the unit needs to survive apply damage step. Assigned damage isn't on the unit yet, there is nothing to heal (at least not from the assigned damage, unit might have previous damage on it of course).