Just look at low level spells and decide what you can realistically cast at something like 8 INT or DEX to aid you in hand to hand combat.
Five best low level spells from each school (as many as you can learn at meagre +2 skill):
Scoundrel: haste, invisibility, charm monster, heal, teleport. (DEX)
Pyro: burn, haste, boost PER, flare/fireball projectile, summon elemental.
Hydro: heal, slow, rain, freeze, summon elemental.
Aero: freeze, blind, teleport, absorb 1/2 damage, invisibility.
Geo: spread oil, bless, armor, AoE knockdown, spider.
Witch: enfeeble, curse, blind, summon a skeleton and destroy summon.
My favourite here would be Fighter/Hydro, with his rains dousing flames to allow movement (which is critical), and freezes and heals protecting own team from damage. He can also take a point in Witch for a trio like destroy summon, blind and enfeeble to strengthen that tactics.
Then pair him up with real mass murderer, on 2nd fighter (a 2-hander preferably) drop 1 point into Pyro and Geo to haste, bless and armor your guys in the early rounds when enemies get closer, and then just wipe them out.
Then you can add a mix of archer/rogue, using arrows from sneak or switching to dagger and backstab. And finish the team with the mage that actually has awesome high level spells. A proper three-school DPS-er, someone that'd for example have all the high-level Pyro/Aero/Hydro spells and maxed INT to use them to their fullest.
