TheCuddlyOne: When you say '+2' you are referring to the THAC0, correct?
Yes, the THAC0 bonus on the weapon (or the + bonus on the ammo).
TheCuddlyOne: Okay, my Tank has a High Quality Battle Axe [1D8; THAC0 +1] I found in the catacombs but I have had no luck finding any other axes to replace it with. I can afford to get him the Faith Killer [1D8 +2; THAC0 +2] from Conlan's Shop. He found one +2 Throwing Axe so I think I'll save that. His AC is -4
You should have found a +2 axe in the upper areas of Dragon's Eye (specifically, the level where you find Conlan's son and where Mother Egenia is imprisoned).
TheCuddlyOne: My Fighter/Thief has Erevain's Broad Sword [2D4 +2; THAC0 +2]. His AC is -2
I actually acquired Conlan's Hammer for my Cleric/Ranger upon my return to Kuldahar. Her AC is -1.
Good stuff here.
TheCuddlyOne: My Fighter/Druid is equipped with a simple Scimitar since I use her for ranged attacks but she only has a +1 sling and regular ammo. I could probably get her The Lucky Scimitar [1D8 +2; THAC0 +2]. Her AC is 2.
The sling will be useless against Yxonumei, since you don't have +2 ammo.**
TheCuddlyOne: My Bard has the Messenger of Sseth Bow [Damage +1; THAC0 +2] and 44 +2 Arrows. She is the only one skilled in Small swords and has the Sword of Days [1D3 +3; THAC0 +3] but I'm wary about sending her into melee combat against Yxonumei especially since she has no armor equipped and her AC is 4.
No worries, the arrows will hit her.
TheCuddlyOne: My mage has the Mage Dagger +2 equipped for the extra level 1 and level 2 spell slots, so I can have an extra Burning Hands and Agannazar's Scorcher spell. She hardly ever uses her +1 sling anymore. She has 5 +2 bullets I've been saving and a Wand of Magic Missiles with 23 charges left.
I wouldn't send the mage in, but I'd suggest filling every level 1 slot with Magic Missile and just keep launching those at her. She may resist them, but some might get through.
TheCuddlyOne: I've been carrying around 2 +2 Flails (Fast Flail +2 and Hammer Flail +2) but no one in my party is proficient with them. I was going to sell them but would it be practical to use them in the fight against Yxonumei? I have one Gauntlets of Weapon Skill.
Warriors only receive a -2 penalty when using a weapon with which they're not proficient, so if you had to, you could use them with a warrior. Other classes get higher penalties (if memory serves, Rogues and Priests get a -3 penalty and casters get a -5 penalty).
TheCuddlyOne: Potions:
20 Potions of Healing (which don't seem to do much for my tank anymore, he'll have to drink them pretty often)
8 Potion of Extra Healing
1 Potion of Regneration
1 Potion Stone Giant Strength - 20
1 Potion of Hill Giant Strength -19
1 Elixir of Health
5 Oil of Serpent Scales (Raise AC by 2 for 6 rounds)
7 Flaming Oil (for throwing)
1 Oil of Fiery Burning (for throwing)
5 Potion of Genius (for Mage)
2 Potion of Mind Focusing (for Mage)
1 Potion Stone form (Sets AC to 0) - I don't think this is useful since all my frontline characters have an AC under 0.
I've also been selling potions that set stats to 18 since my frontliners all have 18 in STR, DEX and CON.
Use the potions of Giant's Strength with your front line warriors. Stone on the main guy (the one with the lowest THAC0) and Hill on the secondary. Mind focusing potions would be useful for any character with low DEX, but since yours all have 18, you're good. The potion will raise DEX top 21, which gives +1 AC bonus, but it may not be worth it for just that. Something to consider though.
Try testing the potion of Stone Form in conjunction with a potion of Mind focusing (the latter will offset the -3 DEX penalty of the former) to see if it improves your AC (the potions affect your base AC, so it may stack with your gear. Not sure, but well worth checking out).
I'd also suggest going back to Kuldahar to grab any other useful potions from Oswald (the Gnome potion merchant). Look especially at the potion of Heroism, since it improves your THAC0, as well as Oils of Speed (if you don't have the Haste spell yet - actually, even if you do, they're useful). Potions of invisibility can be a lifesaver if your character is low on HPs and needs to escape battle long enough to heal up. The key here though is to make certain that at least your melee fighters are hasted. Extra attacks per round make a huge difference.
**Not sure, but the ammo and launcher bonuses may stack with respect to the requirements to hit (i.e. if the sling is +1 and you have +1 bullets, it may qualify as +2 to hit. Again, not certain though).