I need some tips for the last battle this is my party:
Me,(lvl17 cavalier DEX 15!)
Jahera
Imoen
Nalia
Minsc
Mazzy

I have beaten it but with heavy loss of life and equipment which i can't replace (red dragon scales, Vorpal sword,Pride of the legion)

Any help would be appreciated.

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De Grader, just pull ALL you guys, esp Imoen and Nalia, down to the lower right corner. Already have cast 5 of the most powerful summonings that you can. I like Elementals, and save Skeletal Warriors for after they get killed by Jon's demons. Be sure to cast Protection from Evil on your whole Party. Kill off the demons one at a time, then go after JI. Remove his Magical protections(I like a combination of Breach and Ruby Ray of Reversal), Protect two or three of your strongest fighters from fire, Have them go down(he will be just above the bottom) and Haste them and have them hack him to death. DO NOT let your mages anywhere near him. Let them pelt away with ranged weapons, or, if you lower his magical resistance, Magic Missle.

Degrader, there's no need to run away. This is really an easy battle.

1. Cast Simulacrum on both Nalia and Imoen. (Now you have 4 mages!)

2. Summon the 5 best monsters you have available. Use these monsters to attack all 4 of Jon's demon allies SIMULTANEOUSLY. Their job is to keep the demons occupied.

3. Cast all of the standard buffing spells on your party. Stoneskin for the mages, Iron skin for Jahiera. Bless, barkskin. Drink the various fighter buffing potions. Cast Haste on the party.

4. Move the summoned monsters up to the area where Jon and his demon allies will teleport in. Move the cloned Nalia and Imoen. a little bit away. Not too far. Just enough so that they wont be engaged by the demons.

5. Start the sequence that will start the battle.

6. Engage Jon with your cavalier. Use your mages to cast breeching spells on Jon as necessary. Engage the demon allies with Minsc, Mazzy, and Jahiera, along with your own summoned monsters. You don't need to focus on only one at a time. You have enough firepower to take them ALL down at once. Have your mages fire off magic missiles, etc. to support the attacks on the demons. However, always be ready to cast those breeching spells on Jon if he tries casting protection magics. Once you've wiped out the demon allies, Jon's existance can be measured in seconds. Throw everything at Jon and he'll go down shortly.

Yeah, I cast Simacrulum, too, but I didn't know if he had it.I also used a Timestop scroll, the first time.

I did it a really weird way. What i did was cast haste before i initiated the battle. Then when i initiated the battle i ran away to the bottom of one of the stairs. Form here i could almost attack the demons one by one. I killed them all, healed up, waited a bit, then launched a final attack on irenicus, taking all fighter potions, making sure i kepp casting breech, e.t.c. Irenicus eventually fell and i completed the game.

Thanks! Moniseur Irenicus you are dead....