Fenrisulfr: level 12 for a challenge (aka you'll need consumables), 16-18 normal
Dophin: Did it at level 20, was mostly normal, slightly on the challenging side, didn't do it early enough to be a MAJOR challenge, I imagine or 18 or something.
Ice Fenrir: Doable as early as 22-24, but you'll need to rez people every turn, level 26-28 is a decent fight, but it'll still take squishies like compa from full to dead in a single turn. SOMETIMES it uses ice breath (rare), if it does, it might seem alot harder than that though.
Flame Fenrir: I dunno, I didn't attempt until level 38, at which point it was not much of a threat...Still takes awhile to kill but yeah
Elemental Dragon: Ditto, I waited until like level 38 and IIRC it was so weak I killed it when trying to go for a symbol attack, doh.
Demonic Fenrir: level 38 didn't kill it during symbol attack, but it wasn't very hard either. It seems to use breath attacks a bit more often (still pretty rare though), if and when it does, its difficulty shoots up as though it were 10 levels higher though. I can kill it with no threat whatsoever without using any consumeables at 38, but if it breaths, it like 1-shots 2 different people despite being over 50 degrees apart from the beast...then proceeds to melee a 3rd person like 2 times.
Plaid Dolphin: Did at level 46, seemed doable, try earlier if you want it to be boss-like.