My memory here is rusty, but pre-buffing (including Improved Haste) and pound (with Critical Strike, not with Greater Whirlwind) on the AntiPaladin (who dispels on hit like Carsomyr), you should be able to kill him in the first round (otherwise, say good bye to your buffs). Spells here are basically useless, since they have loads of immunities (unless you throw in a Planetar and an Elemental Prince), the role of your casters should be to either attack with summoned weapons and/or cast summons/buffs. Sola is immune to the time stops, try to get the enemies to attack him, and during eclipses have them chasing Sola by doing some microing. They also are stupid against Protection From Magical Weapons.
Try to give the Cloack of Non-detection and loads of Invisibility Potions to Valygar, then backstab them (Staff of the Ram, if you have it and the skill). It works great against the Kensai, Antipaladin and Cleric, whereas the others are immune (Barb) or have Stoneskins (Sorc, Druid). Fighter/Thief + assasination was my favorite tactic, but alas