First of all, Edwin seems to have a major problem. His AC is 9?!?! You need the robe of vecna and the amulet of power from SoA to lower his AC and spell casting time. Do you keep up a stoneskin at all times. It's an essential spell for spell casters to protect them from fighters. Mirror Image is also like another stoneskin that lasts less, but can also take some spells as damage for you. Protection spells such as spell trap (using it from the staff of the magi is enough) will help you as well. Buffing up your spellcasters is very important. Buffs that I often use are stoneskin, mirror image, resist fear (sometimes), improved haste (from kangaxx's ring, which you missed in SoA), protection from evil if fiends are around (staff of the magi provides this. This staff is in SoA), then there are those spells that make you immune to some weapons (protection from magical weapons, mantle, improved mantle, absolute immunity), and protection from energy. ADHW is a great attack spell (level 8). And keep in mind that Time Stop is [i]the]/i] spell
You can easily get Sarevok to -9, or at least the double digits. Have you gone through Watcher's Keep and upgraded the items you found there through Cespenar? You can get some very powerful items to lower your AC and boost your resistances dramatically.
Are you using your HLAs? Abilities such as greater whirlwind are very powerful. GWW gives you 10 attacks per round, which is enough to damage Mellisan in a short amount of time. Hardiness will protect you a lot as well. There is an option through the Wish spell which allows you to cast hardiness on all of your party members.