Uulan Guardian

Trample, reach
Ambush- When a creature with power equal to or less than this creatures power enters the battlefield under an opponents control, you gain 5 life and that creatures controller is dealt 5 damage. Damage dealt in this way can't be prevented or reduced.
{g}{w}: Each opponent gains 4 life and Uulan Guardian gets +X/+X until end of turn, where X is half the total amount of life gained this way.