How about if you made it slow and lumbering, resilient so it can take a hit but is easy to run away from. Its terror attack should be a powerful zone based attack (its own zone, a bit like the earth elemental or even neighboring zones (slightly lesser strength)).
I guess I was trying to imagine a large scary creature that noone wants to be left close to near the end of the round as that's when it will be most powerful. You can try and swarm it to take it down, but theres a pretty good chance they'll die if you aren't successful. However, even if you take it down your worst dreams will come back to haunt you again real soon.
Would it work to make the attack compulsory? i.e. all creatures within one zone range take the pain? That way you can't use it to guard your mage, but its very effective anti swarm or can assist a swarm from another side of the arena.