If you’re underprepared it certainly is an extremely tough boss. Especially since a couple of his attacks can wipe a single character (rock drop) and your whole entire party (demon rush) I never made that mistake twice.I had to restart my game here. It was my first proper JPRG so I only had one save and you couldn't get out of the temple. My characters were not a high enough level to beat the boss.
Haven't made the same mistake since (multiple saves for the win).