Just give him items so he can cast at -10 casting speed, use Improved alacrity to launch a battery of dispelling spells at all your characters, then time stop again and improved alacrity again, and when the second time stop is over your party just receives on the head every offensive spells he has (and only his rain of terror is enough to wipe your party if you are not protected from magic damage)... so he would win all the time, and your only option would then to reload and try to find the ultimate french cheese to beat him...
In first i thank inferno for this great mod ( and Hlid for the weidu version )
It is not a bug ( i think...) i describe, but i am suprised that Melanthium don't cast any spells of abjuration to remove protections against fire, cold , magic etc... ( remove magic, khelben, breach, ruby ray etc...)
In fact with this tip the fight is easy just because you only must focus his attention ( in order to avoid maze or worse ) with a fiend ( the better i think is to memorize 2 or three of level 8 ) and wait his shield of oblivion falls ( the fiend will kill him )
The best is a berseker if you have it one
In fact he used these spells after his first death ( after the dialog ) ( but ONLY one spell: nine level of abjuration )
OK i agree that the fight would be impossible to win in SOA if he had these spells and according to the background he is a so powerful sorcerer that he needn't remove spells of protection of his ennemies.
However a powerful sorcerer would wonder why his powerful spells don't manage to kill you and would adapt his strategy.