[Considering his most recent run-in at the school, he's (kinda???) looking for you (or at least Aramis is looking for Arthur but whatever he'll just go to you) and he's quick to actually fight the stupid thing once he sees Merlin having trouble]
[He isn't in time to actually get the thing beaten, but a gunshot does assist in getting the thing to fucking leave, at least, and he's going to him quickly, offering a hand to help him up]