Mell can occasionally get stuck at the magic barriers because he can't get close enough to them to initiate his "fear sequence".
Mell doesn't want to approach the magic barriers when enemies are still firing at him, even if they're not directly visible to you. Using the bow, you can aim precisely and charge up shots to snipe the enemies that are just beyond the camera's range. Eventually you should kill them all and Mell should proceed.
Restarting the game and loading from an earlier save can also cause this problem to resolve.