This post may contain minor SPOILERS
Normally, you should dual from Fighter to Thief to get more HP (you don't gain alot more after lvl 9). If you dual from Thief to Fighter, you will have a weaker thief with less skills (since you won't be developing them anyway). Extra Thac0 and Extra Weapon Proficiencies will be useful, go for it. Don't dual-class before you have maxed a few important thief skills :
- Detect Trap
- Lockpicking
- Hide in Shadow/Move silently (if you like to backstabb/scout) or Pickpocket (can give you items for FREE in some shops) or Detect Illusion (Underrated skill. Will dispel any kind of illusions in 1 round only!)
A good level to dual-class is a level when you gain an extra weapon proficiency :
- Level 8 (4 weapon proficiencies)
- Level 12 (5 weapon proficiencies)
- level 16 (6 weapon proficiencies)
I would go for level 8 in Shadow of Amn, level 12 or even 16 (if you want to max ou different skills) in Throne of Bhaal.
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Dual-class = Start as a class X (Thief) and stop your progression at level Z to restart as a Y (Fighter) from level 1. When the level of your Y class become superior by 1 to the level of your X class, you'll regain the abilities given by the X class.
Multiclass = Advancing in 2 (or 3) classes at the same time. The experience given to the Character is divided by 2 (or 3).
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Now for your party :
PC - Thief (possibly Thief/Fighter)
Minsc - Ranger
Aerie - Cleric/Mage
Aerie is a good spellcastress, since she can use Arcane and Divine magic. The only problem is that you will gain high-level spells (level 7+) really late with her. Since the maximum level for cleric spells is 7, she can handle the Clerical part fine, but I would add another mage.
As for Minsc, he's a good thank, and a funny one. I say keep him. He doesn't have grand mastery and high hitpoints like Korgan, but at least he's funny
. If you want power, go for Korgan. He's a 4-foot high killing machine.
Okay.. you may want to add :
- A support character (Bard, Archer, Druid).
Haer'Dalis makes a good 2nd line fighter, as well as a backup mage. He can also pickpocket, so you can concentrate on other thieving skills.
Mazzy is hands down the best ranged weapon using NPC in the game. Equip her with a Short Bow, and she's real minigun.
Cernd can be used as a druid spellcaster, or as a frontliner (when in Werewolf form).
Jaheira is useful both as a spellcastress (druid) and as a frontliner (fighter).
Valygar could be useful with the right longbow and a backstabbing weapon, but since he's a worst archer than mazzy and a worse backstabber than your main Character, he isn't going to help alot.
You may also want to add :
- A single-classed mage (or dual-classed) to take care of high level spells
Edwin is hands down the best mage in BG2. Because of his amulet, he has plenty of spells slots (+1 per level because of his amulet, +1 per level because of his Conjurer speciality). The only downside : no divination spells. That's not a problem if you keep aerie.
Imoen is a decent thief (useless, since your PC is a thief) dual-classed to a mage. As a mage, she can reach high-levels spells quickly. The only problem : you will have to hurry to rescue her if you want her to level-up. Otherwise, she will be way too low level when you pick her.
Nalia is a clone of Imoen. If you do not hurry to get Imoen, Nalia is a wise choice, since she will gain experience while Imoen will still be in spellhold.
So, here is what your party should be looking like right now :
#1 - PC (Thief/Fighter dualclass)
#2 - Minsc/Korgan
#3 - Aerie
#4 - Nalia/Imoen/Edwin
#5 - (Support Character) - Optional
#6 - (Support Character) - Optional