The vine strikes Susan and he stumbles but he pushes on, needing to get to his brother. Of course Lauren can look after himself, but when it comes to magic he's the firepower (literally) and he can't just leave Lauren unattended. Especially not when everyone else's plan seems to be to grab onto the lion or throw daggers and pokemon moves at it. Why would you do that when you can just use magic? He isn't certain of exactly how strong the vines are, but he knows he has magic that can pull things easily himself, and so he summons up a telekinesis spell to try and enter into some sort of strange tug-of-war with the vines over Lion, pulling with all his magical might to try and get the beast away from the monsters attacking them.