Buckthorn will shift one square up and right (northeast) to get away from the Simalcrum Simori. He will then shoot at Simalcrum Simori using Pinning strike (Rolls two 33 to attack). Assuming they hit, that would be 27 damage for the first arrow and 16 damage for the second arrow and the duplicate Sim would be immobilized until the start of Buckthorn's next turn. He will then make a melee attack using his third arm and reach from his elastic skeleton (30 to hit, 9 damage).
Note - if Buckthorn is hit again this round, he will use Second Chance.