Simple solution: Lower the damage that Flaming Aura deals, and edit Balor to balance things. For example:

Balor Demon King, Duke of Hell, Tempter and Tormentor

Repelled: Protection, Restoration
Denied: n/a
Special: Health_Regen -2, Spell_Regen -1, Luck -1, -5 Resist_Cold (which means maximum resistance to cold is 90% instead of 95%)

Granted Spell(s): Flaming Aura (medium level), Rage (medium level), Vitriol (medium level)
Altar Effect(s): Remove Curse, Enchant Weapon (medium)
Holy Relic(s): n/a

Just remove the underlined parts, and lower flaming aura. That way, aura still does some damage, though less than before, yet health regen and cold resistance are no longer affected.

that's not a very good idea ... his stats are like that for a reason ..

Maybe so, but it'd certainly balance things more given FA's damage was decreased...

Aura has been fixed yes, there is a stronger aura spell though i think

An aura spell? Never came across any kind off spell that would work similar to Balor's flaming aura...

His stats are like that because he has good spells and altar effects. I don't see any reason why anyone would pick him otherwise. I mean, all his stat changing things are negative.

Stefan didnt change flaming aura to be get at 1 max grace
i played the game the mudlib was taken from its same there stefan just didnt change it but they had a script which doesnt allow to cast it multiple times

That's because Stefan unlike several of you realize flaming aura is quite a fair spell, the mutiple casting was just a bug that needed to be fixed, he unlike most of you isn't stupid enough to drag on and argument that doesn't matter because it didn't need to be changed from the start, only fixed.