This is my party which is working out pretty well for me (killed Kangaxx and Frikraag at about level 12/13)
Damien - Elven Beastmaster
This was actually an experiment which worked fine as I now use the rangers stealth and Staves as weapons Also he's able to use bows - a normal ranger is better though
Rikard - Elven Sorcerer
As you said yourself Sorcerers are invaluable for any team
Iason - Assassin
He's a pretty useful fighter or at least when he's not hit (he has the lowest XP because he dead so often) can kill any mage by backstabbing (as long as they don't use true sight) And kills of liches with one press on the daystar button
Jerome - Undead Hunter
Carsomyr rox in combo with his 127% Fire resistance
Ideal against fire using monsters and undead
Dolgan - Cleric of Helm
He's where i get my True Sight spells from
Also a great healer and a moderate Fighter (with Crom Feayr)
Mikaël - Bezerker Dualled to Mage (level 8)
Bezerker Rage can come in handy against imprisonment spell users
Also backup Mage who has the spells not so often used and not memorized by my sorcerer and ofcourse a XP-Boomer with spell learning
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The Rocks show the way