Alright, I've finally gotten to the imporved fish city. My party consists of a Ranger/Cleric 11/12 and Imoen 14. I have mostly +2 weapons and a +3 staff. First, I want to know how people beat the Minor Avatar - every time I came near him, he casts the equivalent of Imprisonment and the game ends. Undeterred, I turned into the Slayer and gutted the fish (not without some help from the Rod of Resurrection, but still). However, I absolutely hate to use the Slayer - how else do you avoid the 'Swallowed Whole' thing?
Second, the Sahuanin themselves. These are clearly enemies from ToB - they're level 14, they receive a +12 to their attack rolls against an AC of -6, they do 20+ damage per hit, they make you save vs. stun with every hit, and they have +1 attack crossbows and +3 spears. So, why are they only worth 3000XP per kill?! Who in their right mind would fight these creatures when you can run around sanctuaried or invisible? (no, I am not suggesting that you make them detect invisibility, then I'd just have to use Prot. from Magic Weapons) Those stupid infinite dwarfs that Sendai sends against you are twice as weak yet they are 12000XP a piece - please make the Sahuanin worth fighting.
Althernai I finished this some time ago and I struggled initially because I am a lazy player, not taking the time to buff. Mind you I defeated the Avator second time.
First set up a trap for the ambush (Strangely that makes sense!) the Avatar and Priestess have waiting for you. I cast lots of skull traps and delayed blast fireballs where the Sahaugin and Priestess appear. I used protection from evil and summoned a pit fiend and other summons. I then kept most of the party away, half the Sahaugin died straight away when I sent Korgan off to spring the ambush. Don't have any party members directly attack the Avatar, thats asking for trouble. Apparently bezerker rage renders you immune to the swallow.
Launch missiles and fireballs at the enemy and mop up with your fighters once under control of the fight.
For the regular rebels with the stun bolts you need lots of summons, mordenkainens swords are great here, they are immune to the missiles and will last longer than Skeletons etc. Focus on one rebel at a time as they will heal themselves if left hurt too long. Careful of having your protagonist at the front as he/she will die if they get stunned early as the Sahaugin all focus on the first NPC they see. I thought Keldorns armour with the free action may have thwarted the stun but it didn't seem to.
I too won't change to the Slayer, only once to open the mind Flayer door, I wanted to keep the collars.