I don't know, I saw my friend one-shot like 90% percent of the bosses with Comet Azur, as in literally entering the boss' room, gulping down the physick flask with the tears that give you infinite FP, and killing the boss before it could even attack. There were exceptions like Melina, but he used ambush shard which was surprisingly damaging, she wouldn't dodge at all, and then once the second phase starts and she does her lengthy dive attack, it was Comet Azur time again, one shooting her. Any boss resilient to magic was dealt instead with Rock Sling. The only ones that gave issue were the last boss, which forced him to respect to a STR build. And I will argue that he didn't play his character at its full potential, he rarely used Terra Magica and/or the Moon spells nor inflicted frostbite consistently, which would had contributed greatly to increasing his damage.
I agree that maybe magic isn't as good with groups of regular enemies, but there are a few spells that truly seem to trivialize most of the bosses.