Castellan Toad Boss' physical hitbox.

Earlier today I was fighting this boss on my corsair who (with the help of some gear) is able to tank its magic attacks. However when I was standing about half of cannon's range away from the toad, when his tongue lash attack hit me for physical damage and I died. I'm certain that this attack is magic, and that I normally would be able to take it without a problem. Apparently, though, when he uses that move, his physical hitbox extends outwards. (I might be wrong though, if someone can offer a better explanation, please do). What I want to know, is how far away I have to be to avoid this boss' physical damage.