First of all, yes, I know fusing is important and I'm well aware of that.

However right now I really need my Jack Frost to simply level up once more, in order to learn the medium ice spell. But dear god, it barely gets any exp at all compared to the main character.

Is there no way to speed up the process?

Are you fighting the strongest enemies available? If so, then Jack Frost is already earning the most EXP he can. Unless...
Experience is divided between your party members. If you bring less party members with you, that means you'll get more EXP for you and Jack Frost.

Ah thanks!

Though turns out I didn't need it after all, just beat the boss king in Yukiko's Castle :D

gameshark

yeah.... the king in yukiko's castle is really no match at all for jack frost. like... not even close really.

Just go into a previous dungeon solo and kill the dudes you find.  That's how it worked for me when I was 80+ or so and needed Victory Cry from Lucifer.  Though I had bonus xp gear with me.  I didn't feel like doing the pringle trick at this point.

You need to go solo into the dungeon starting from Sauna dungeon.Deal finishing blow with persona you want to level up.I know it is hard especially if you are in first playthrough but after you face same enemies several times,you will get some hints on how to kill them without giving them any chance to kill you.

The tips below is useful when you want to level up main character:
If you are already in September,there is Haikara shirt which you can get from oldman at riverbed(neeId Guardian fish).In December,you can get Musashi shinai(2 Guardians required).Equip both of them and go solo.You will reach at least level 90 before you fight main boss in Heaven dungeon without wasting time fighting respawning shadows.

The best way is level up the social link and refuse it.

Seriusly

I guess you are at start of the game in the first dungeon(or second) there aren't much more option.

If you really can't do it just ignore the spell, while its a nice boost you will be able to beat any enemy even without it. It will take more time but just a fraction of the time needed to level up a persona.