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.

All bosses that extend their range have this. (Halloween Toad, Cat, etc)

You have to stand about 25 pixels from the portal and it won't hit you with that attack.