Fight him from biggest range you can. His tree throwing skill can stun you from large range, and leave immobile for almost 2 seconds.
If you want to engage, use your W to tank on three throw. And run immediatelly after.
If he is under 60% hp, hit him from range to make him jump, get out of vision field of his (a bit smaller than your vision). Count to 3, then go engage back. He will land by the time and you will dodge massive damage and stun.