[teardropmina]

I don't think there's summon limit for Melissan. If you give her break to summon when she's low on goon counts, I think she will summon more regardless of how you lower the amount of her goons. That's why you should focus on Melissan and efficiently beat her to low HP every time she shows up.

you can use the Project Image cheese to get more summons.

the fatique problem -- cast Wish from scrolls, and pick the party rest wish (you can actually save the game between battles and so you can reload to get the option if it didn't show up when you cast it).

IIRC (been a long time since I last fought the standard final battle), your party with 5 summons (no need to use PI cheese) is capable of winning the battle, but quite a tough fight, and casting a Wish spell is almost inevitable (well, for me anyway). If you find the battle simply too tough, you can always level your party some more (get Sarevok cross lvl 30, and Keldorn and Minsc around 28-29).