I tried the stairs and such, but I too had problems with my guys getting out of LOS and too far away to gather with AF. Now I just tank him up top in the middle.

Best tips for my team:

1. Set your camera distance to max. Being able to see everything makes all these fights a lot easier.
2. Don't bother trying to remove the frost debuff until he runs to the forge. After the stun, I gather everyone up, run them around behind a boulder/swing by the stairs on my way to a clear spot where there is room to move from the rest of the boulders.