I'm stuck on the final boss of Throne of Bhaal, I guess because my party is kinda shit (being evil): Wizard(Necromancer) - Dorn - Sarevok - Korgan - Imoen - Viconia , any advice? I reached the third phase but then i'm getting oblitarated. Why there must be 1 planetar on screen only
My friend, you have a party with 2 arcane spellcasters and three top-tier martials including Sarevok's Deathbringer Assault letting you bruteforce encounters if you're persistent enough (200-400 damage strike can suddenly win you any fight). A "vanilla" evil party (just the original BG2 evil NPCs and maybe a wild-card Neutral or the Neutral-Good-But-Actually-Don't-Care-Imoen) is one of the most cookie cutter power composition, as the NPCs have great stats, good kits and are generally solid, if not absolutely outstanding.
Viconia is great too, you just have to keep in mind that she's best as a support / damage soak right now. All that magic resist she has means that you can let her just soak damage, there's a belt to get her to 18 CON, and a plethora of Cleric buffs to make her untouchable. Just make her catch aggro, regular AI doesn't really care if Viconia will hit the enemy or not. If you want Vicky to deal damage, give her a Sling of Seeking and buff her STR to 25 using Clerical spells (20+ damage with each bullet, and Vicky has high innate DEX) or just conjure Energy Blades. Yes, Clerics are way past their power spike, but you still want them for the Deva and immunities, most importantly to level drain, but she can also cure poison if needed (Mariliths deal poisonous attacks).
IMHO you probably need to just dispel the defenses as they appear (Breach, Spell Thrust, Secret Word, Lower Resist, there's a wand of Ruby Ray of Reversal at some point, etc.) - you
need to do that, for instance, against the Mariliths, because they like to use Protection from Magic Weapons on top of their own natural immunity to normal weapons, so until you stack up your dispel game, you can't hurt shit. Use every single consumable you can possibly need, and don't shy away from using any and all wands.
You have a heavy martial party / frontliner, so IMHO you should just focus on making sure they can cut through stuff by dispelling shit. Martial DPS is the best DPS in the game, you just have to enable it.
Also use Khelben's Warding Whip on tougher things because it makes sure recasting defenses doesn't work.
Once Amelissan shows up you should go full speed ahead on destroying her. Again, use Viconia to draw away her escort if you need to, with enough buffing and natural magic resistance she can last for longer than you give credit for - she doesn't need to do damage, she just needs to soak. Three fighters with Greater Whirlwind should hack Amelissan to pieces. Stack Hardiness on your fighters to help beeline for the kill. Use your Rod of Resurrection instead of wasting time on healing spells during the fight, it's virtually an instant heal for anyone who wasn't chunked. Either of your mages can also, actually, help the frontline by simply soaking damage with PfMW, in case you run out of spell slots (use Project Image / Simulacrum to preserve them!) and ideas.
Time Stop doesn't work, so don't use it, it is no use in a party environment.
tl;dr: SHE CALLED DOWN THAT THUNDER, NOW LET HER REAP THE IMPROVED WHIRLWIND