Jax, Grandmaster at Arms

Real Weapon—Whenever Jax, Grandmaster at Arms enters the battlefield or attacks, stop all damage that would be dealt to him this turn by up to one target creature. Whenever damage is prevented this way, put a mastery counter on Jax. Whenever Jax triggers double strike from his abilities, remove all mastery counters from him and put a +3/+0 counter on Jax.

{1}{u}: If Jax has exactly two mastery counters on him, he gains first strike until end of turn. If he has three or more mastery counters