So my redone party rocked the beginning and middle of the game. I think making sure the wizards had a Constitution of at least 16 each really helped out, since it resulted in them being able to take a few hits in battle instead of being glass cannons. I even got through that first orc attack without any problems at all with the new group. I also took on the orc stockade with no problem as my characters were level 7 by about that point, like
VentilatorOfDoom had suggested. The key there was that I was able to beat the dungeon with the spider queen, then found the additional level beyond that encounter and came out rich in XP and gold. I think I may have used a confusion spell on the dragon, then hit him over and over again with fireballs from some wands I created.
The middle of the game went fast, too, since my wands of fireball continued to be effective all the way until I battled against fire giants, where I had to switch to ice storm wands. I didn't really run into trouble until the end of the game where damaging spells became less important due to enemies with crazy amounts of hit points and you had to hope for enchantment magic to work such as dominate monster, in order to have a chance. If I could avoid getting ambushed I ended up with a pretty standard strategy of retreating to a choke point, having a wizard cast web at the choke point while another cast haste on the party, follow that up with a silence spell from my priest past the webbing, then have my wizards conduct long range bombardment on everything past that point as the computer AI would go dumb at that point and have its opponents loiter. It was generally pretty effective.
But the game liked to cheat. There were way too many encounters where the enemy would just pop in out of nowhere, surround your party from all sides, and get an automatic surprise round to boot. Damn cheaters. With the enemy being so powerful, they would likely win with two free consecutive actions, usually by slaying one or two of my wizards before I could act. Once you get to 15th-20th level, this becomes a standard issue in all editions of D&D, where everyone is so deadly that the side that wins initiative is often the side that wins, so I guess points to the game for properly simulating that tabletop experience. Or simulating play with a cheating bastard DM.
I also misunderstood something towards the end of the game. There is a dungeon inside a town and a tower with an undead ruler. I got confused about which dungeon was meant to be the last, and I ended up beating the game without completing the last side quest, so it was a bit jarring for me when I rescued an NPC and then got the end game scene. Oops.
Ah well, I doubt I missed much anyway except more gold I really didn't need.