Mogli? you got owned by that little kid in the Jungle Book? XD
like the others said, you can use a magic mirror, absorb/null fire personas, null/strong/dodge fire accessories, or you can also just make sure you have marakukaja cast. other than that, all i can think of is to keep your health up. --- Man has only three essentials to hold on to in the world: life, death, and time. He must take care to not waste any.
I fought it at level 47. Level 40 is a bit low level to fight it. A good persona to have is Thoth if you have maxed out Hierophant S. Link, since you will learn Mediarama which will heal everyone by a little more than half their Max HP. --- For those without dreams, may you die in peace...
Putting up mirrors is one of the worse things you can do against that guardian. It will spam you with megiadola every round more than likely if you put up mirrors.
I just used a persona that could null out fire and had some healing spells. Junpei is a good idea too since he resists fire...just put him on full assault. You'll still get bombed with megiadola occasionally but you'll have no problem against the fire attack.
This boss was giving me a hard time, and using magic mirrors simply wasn't doing it for me, so I tried a different strategy that worked quite well. I leveled Saki Mitama to 42 so he learns Mediarama, which is great for this fight considering the amount of damage Sleeping Table dishes out. I used Yukari, Shinji and Akihiko for my party, having Yukari and Akihiko both on Heal/Support with Shinji doing most of the damage dealing. Just keep spamming Mediarama, and with Yukari's healing spells and Akihiko's debuffs, this boss wasn't too difficult, just lots of healing. The magic mirror method is probably easier but it's just another strat for this boss. --- Lesson #3: If you see a stranger, follow him.