Korgan is an excellent tank. Have him at the front with the best armour. Do same if using Mazzy
Keldorn can be back-up fighter and attack the monster's flanks as Korgan engages them. Minsc will do for this too
Haer'Dalis' spin abilities are good but keep him as an archer unles monsters are relatively weak
Likewise with your Thief - use Shapeshift to flank medium beasties and backstab tougher ones while Korgan engages them
Viconia/Nalia/Aerie/Jan - use lots of Summoning spells (not the crappy monster ones, use elementals/stalkers/aerial servants)
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