The problem with Odin and Behemoth is that even if you're there RIGHT when they're pulled, and you're on your feet the full duration, you'll still most likely end up with Bronze or Silver medals especially given the level disparity. You'll most likely never hit for full damage, if you hit for damage at all, since he's over level 50 (55 or 56, I think). Those FATEs are very, VERY picky on doling out rewards, and party progress seems to be the only real way to get a higher reward. Also just being able to SEE him is enough of a problem for most everyone there, so all you're really doing is putting another character model in the area to load that's not actually adding anything meaningful to the fight and possibly risking them losing their target lock.