I'm at the end (I think) of chapter nine. My problem is the boss at the end (I think) there

Dodging his attacks were pretty easy, and I was able to get rid of the main body no problem, but I can NOT hit his arms!!!! He just won't hold still! I managed to hit him a few times with guns and magic...like...four times in ten minutes x.x

...aaaannyway, is there some super neat trick to doing this or do I gotta be a little more patient with him and his flailing arms?