The gluttony fight was intimidating and well hinted at, to be sure. Look, I for one liked it, and I played it several times (only later realizing that you can use Smite from the healing school of magic to deal good crits on it as it's supper effective), and idk what you can do to make it more challenging.
I think it's fine as it is, because the alternative of conveying how powerful this devil can be vs how weak the MC is, is to have a scripted "one shot" a-la Lich King battle where he wipes the raid. And while some may prefer that, I think it takes away from the agency of the player and would make it feel less impactful. You can uptune the damage he deals, if you want, but you have to be careful anyway since the +25 Hp stat increase from Ayna isn't gonna count for a lot if it ends up being 1 attack from him.
No one wants a scripted one shot. You played too much WoW.
The +25 HP and +25 MP gain from Anya happens earlier in the story. The red crystal that the MC absorbed has a mind of it's own and forces it's power on the player giving +300 HP, +300 MP, and a good boost to all magic skills.
Personally, I would have preferred the game giving me a choice as to accepting the power or resisting. Resisting would have made the fight very difficult but also a much bigger accomplishment.
I personally would have also preferred having a choice as whether or not to flee with the others, or to stay and fight gluttony on my own. If the player chose to flee, it would end in 'game over' as the college would have been overrun.
Both choices were forced on the player.
So if the player had the option of resisting the red crystal and not accepting the power boost from it, then I agree, nothing really needs to change. With the power boost from the red crystal, the fight with gluttony was too easy, even though it took a long time to wear down his HPs.
Don't get me wrong, I liked the gluttony encounter. Gluttony was very scary and intimidating! I just thought gluttony should have been more powerful for a devil and put up a better fight.
Not everyone likes a challenge though so I can see your viewpoint.