Sherwood feels the bow reach it's full potential, the pseudo magnetic draw reaching the point where it's physically impossible for him to create more tension. Now he looks to aim, seeing the beast's head is hidden behind it's arm as well as it's chest. Not that the latter matters, given Grimm must not have hearts. Sherwood would assume.
Not wanting to risk the arrow not piercing through the hand to hit the head, Sherwood shifts his aim to the shoulder of the Grimm's left arm. With this much force behind the arrow, he should be able to disable the arm at the very least, giving the Grimm one less thing to hit his teammates with. With it frozen in place by someone's ice dust, Sherwood had been too focused to notice who, he doesn't have to compensate for movement. Not that it matters, because as his shooting is true...
The hit is inevitable. The arrow fires out of his weapon, propelled by the force of two magnetic poles reuniting in a fraction of a second. Air warps around the solid iron arrow, being pulled along with the arrow to slam into the target as shock wave while the arrow pierces corrupted flesh.
Sherwood doesn't break his stance to see the result of his shot. Proper form demands it, and per the mantra of his mentor, proper shooting is an inevitable hit.