I have played a similar party to yours all the way through TOB, except my PC was a fighter dual to mage, I had Keldorn instead of Minsc and Jaheira instead of Valygar.
Anyway, Keldorn and Mazzy were my tanks, but I made Mazzy the general tank and sicced Keldorn on enemy mages (for obvious reasons). Mazzy performed incredibly well in the front rank. She needs STR enhancement, and then she destroys quite well. I made her a sword and shield fighter for best AC, and only had her switch to the Gesen bow in a few situations (she is a great archer but I wanted her right in front).
I think the big problem is her proficiencies (although it did not end up a problem) - there are some good short swords, some with some good immunities or abilities, but still the best weapons later in the game are not short swords. So as she gained levels, I gave her first a proficiency in Katana, and gave her celestial fury. She did VERY well with it. Even with only one or two stars she hits all the time. STR enhancement helps. She levels up quicker than Paladins and Rangers, anyway. The I gave her next two profs in Hammer, and gave her crom faer. Then she really kicked butt everywhere. (of course anybody kicks butt with crom faer) so basically i'm saying don't worry about her proficiencies she's tough no matter what she uses.
She has a lot of HP too. I gave her: the red dragon scale, the dragon shield, whatever helm, a ring of regeneration, and a ring of whatever, periapt of poisson protection: AC of -12. She had great fire resistance so I could run her in to battle and then shoot fireballs in after her and she just keeps fighting. Good abilities, good saves, good personality. Not quite as good a fighter as Korgan, but she is a good girl and says a lot of nice supportive things to you during the game.
Anyway I hope you have fun with your game

May you walk on warrrrm sannd....