Until now I only used warriors, thiefs and once a cleric and a monk as main char.
I want to use a gnome fighter/illusionist to play through BGT (only used vanilla BG1+2 before).
Until now I only used a mage in the back row to dispell enemies and cast damage spells.
So finally I want to use any char with arcane magic as main char.
But a pure mage is still a bit to fragile to me in the first levels so I want to use multi class because I never did this either.

If you have a fighter/mage:
What do you do when you prepare yourself for a hard fight?
What spells do you use to protect yourself and your party and what do you cast agains the enemy?
I was always afraid of using spells where the target gets a saving throw, because I assume you can defeat weak enemies without spells and strong enemies will make their save always. (I don´t know because I never tried ;-))

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How to fight enemy mages?
I think especially of liches and a late fight in a tree.
My standart proccedure in BG2 was:
-Keldorn used his dispell ability and a mage cast breach.
This has one problem: keldorns dispell also removes my onw protection spells and sometimes the enemy gets invisible or makes a copy of himself. Then I cannot target him for breach. Having a third char casting true seing is one idea (do mages and clerics have this spell?). If the enemies start with time stop and cast something else I can only hope that some chars survive this and are able to do anything at all afterwards.

I try to use weapons with additional elemental damage (long live the fail of ages !!!) Very few enemies are immun to magic weapons and all elements at the same time. Once I fought a lich. I cast protection from fear before, but keldorns dispell removed this as well. So most chars ran around like chicken after a few seconds. I only defeated him because Anomen had the fail of ages. He attaced him doing 1 point of damage with every attac and preventing him from casting. This continued until some enemy pretection wears off and other chars recovered. Then I killed him fast.

My standart party was:
BG1: me, Imoen, Minsc, Dyneheir, Branwen, Adjantis (khalids and Jaheiras whining were too much)
BG2: me, minsc, imoen (joshimo, nalia or jan until then), Anomen, Keldorn and somebody else (Jaheira mostly)
Usually I avoided Arie and jan. Just looking at their HP made me fear for their life if they ever get hit.

Edwin and Viconia may be great chars, but my reputation always goes up like crazy fast.

I do like warriors most, then come thiefs and clerics, but if I tell others often enough I do, maybe I really use an arcane char one day. But don´t expect me to solo the game with a sorcerer.

Keldorn's dispel-magic is powerful so don't cast it near your own party.
Make sure to lead with that when your party is still at a safe distance.
If you're fighting in a close area - don't use it. Rely on breach spells and similar to take down enemy mage protections.

As a fighter-mage you can cast stoneskin, protection from normal/magic weapons , protection from fire on yourself, walk into the midst of the enemy, cast fireball (keeping the rest of your party with ranged weapons or giving them protection from fire too) and then melee with the enemy, with them being unable to hurt you.
Have your cleric cast death-ward if the enemy can cast death-spells.

You have to watch for enemy mages using dispel on you too, but the number of times that succeeds is rare enough that I don't worry about it - just make sure your cleric/mage has a spare 'remove fear' / 'protection from fear' as that's the spell that usually undoes my 'brilliant' strategies if I don't have it lol

Later in SOA and TOB you can get contingencies and spell-triggers to set up cool combos.
When you get the HLA you can choose from both fighter and mage ones (if you multi-class, not dual-class).
So you can add whirlwind attack (and more importantly Greater Whirlwind Attack) to your melee-wizard.

Thanks.
But the problem is, that the hardest fights (liches, tree irenicus)
are in a small area where all chars are close together.
Maybe keldorn uses true seing first and I hope that this makes the enemy a legal target for breach.
You cannot target invisible enemies.
Whats the best way to get rid of things, when the enemy makes a copy of himself and disappears (spell name forgotten)

My fighter/mage wants to spend points for bows (range wins in BG1, and often in BG2 too) and to dual wield axes and hammers. There are some good axes in early SoA and the best weapon is a hammer.
I think the staff of magi is better for Imoen in my party.

I played NWN2 a lot lately so one more question:
Gnomes can use big weapons or dual wield normal ones in BG?
In NWN they are smal chars so they can only use small weapons.

See here for protection spells and their removal. You just answered it yourself: True Sight/Seeing. Provided you don't want Imoen to actually do any damage with it. With only 9 STR, she's hardly ever going to hit anything, but the protections, dispelling and invisibility make it a formidable weapon, nevertheless. The Bow of Tuigan or Gesen is always the weapon of choice for Imoen. Weapons are not dependent on race in BG/BG2, only on class, so yes, a Gnome can use anything that a human of the same class can.

You don't use the Staff of the Magi for damage. It's only +1 to thaco and 1d6+1 damage. You use it for all of its ridiculous perks, none of which involve melee damage and most of which are great for Imoen.

Fighting against mages is super-easy with Keldorn, but only moderately harder without him if you have the right setup. Most enemies before late-game don't require True Sight; Oracle or Detect Invisibility will work fine. (If you have Keldorn, though, you might as well use True Sight on everything.) Once they're visible, you just have to determine whether Breach is what you need, or whether Spellstrike/Spell Thrust/Pierce Magic is what you need. Breach is for combat stuff like Stoneskin, Mantle, and Resist [Element], while the others are for Globe of Invulnerability, Spell Deflection, Spell Turning, and Spell Trap. One thing to beware is that often liches and other enemies will have Spell Shield in a spell trigger, which usually just says "Shielded" without telling you what it actually is. Spell Shield absorbs the first magic-attack type spell you cast at him, so you'll need to either hit him with a minor thing like Spell Thrust to get rid of the shield first before your Spellstrike/Breach, or use Dispel/Remove Magic to remove the shield. After their various protections are gone, mages all go down very quickly. If they still have a Mirror Image up, don't bother with a big divination spell, just use Magic Missile to get rid of the clones.

Keldorn can pretty much just cast Dispel Magic and break everything, though, without extra need for Spell vs. Spell shenanigans. At level 10 he can solo almost any lich or mage in SoA. Dispel Magic will NOT get rid of Spell Turning or Deflection, though, so if you want to hit them with damaging spells you'll still need to break those.

The dispel-on-hit of the Staff of the Magi is amazing. (It's MUCH stronger than the similar effect on Carsomyr.) That's why you're hitting people with it instead of using another weapon or casting a spell. The damage is not the purpose, the dispel is. You don't need 3 attacks per round at subzero thaco for that. One strike a round is plenty, especially since virtually everything you'd want to hit with it has poor AC, and if you're invisible that one strike is at an extra +4.

I agree absolutely. Although the damage is not the purpose, it still requires a successful melee attack, and that was my point, since you stated that none of the 'perks' involved melee damage. This is not so. No hit, no effect.

Is there a difference between the dispell on hit of the holy avanger and the staff of magi?

As written before, I make a fighter/mage, so my Thac0 will be good and I have many attacs per round.
I thought of: First 2 points bow and axe, than 3 points dual wielding, then 2 points hammer.
It takes some time to get the staff and I have to kill the ones who have it first.
The enemies I want to dispell have a bad AC.
So a fighter without profiency but with 2 attacs per round will have a better hit chance than a pure mage, I guess.

Anyway its not good to plan your char around a few very powerful items.
First, most first time players will not get them because they do not find it or you cannot defeat the enemies who have it.
Non of these items is required to finish the game.
Second, if you are good enough to know were and how to get the best items, you are usually good enough to beat all enemies without it as well.

A hit from the Staff of the Magi pretty much dispels absolutely everything. Carsomyr dispels magical weapons automatically, but for other things it tries to use a level-vs-level system like the Dispel Magic spell uses. Unfortunately, because the sword is the origin of the dispel effect, its level is always 0, so certain things like Fireshields or Stoneskin never get dispelled because their casters are always above level 5. It still dispels some of the nastier stuff, but it will never dispel everything like the Staff does.

Presumably this is an oversight, and I'm sure there are mods to change Carsomyr's behavior to be more like the Staff. It's an extremely simple fix.