Recommended party
Have a barbarian and a rogue in the front rows and a cleric and conjurer in the back rows. The conjurer is useful against the corrupter, because he summons four corrupted clones of the party, and the conjurer can instantly kill a summoned monster. He can also summon some monsters to help protect the rest of the party.
Always have a cleric and conjuror. They can both resurrect party members. A warrior is the best melee class because of his leadership skill. Get "Hail Of Arrows" when it becomes available for your rangers. Use it with a bow and arrow with "venomous" in its name, then attach an "orb of toxicity". The toxic damage adds up. The conjurer helps with his "Banish" attack, making fights against neophytes, acolytes and the corrupter easier.
The warrior's "Cleave", ranger's "Hail Of Arrows" and barbarian's "Whirlwind" are a strong combination.
Try a party with a barbarian, cleric, ranger, and mage. The battles never last long enough to for conjuration. Use area attacks, hail of arrows, with the ranger, and the lightning attack that hits everyone with the mage. Then, area with the barbarian and the cleric. Most fights in Standard mode never get past two rounds. Use Boast on the barbarian to delay someone's actions and cause another of your characters to attack in his place. When I need to make sure someone does not attack, use the Pin on the ranger, and Frost the mage. This will usually stun people. Get equipment that has a 10% chance to kill a target on three different characters. When area six opponents, usually one of them dies. This also works well if you have a poison bonus on your weapons.