Okay but you nearly use any offensive spells. Should I keep them to Aerie then?
Offensive spells, i.e. spells that actually deal damage, are not very good. If you just hit things with your sword, you'll deal a lot more damage the vast majority of the time.
The exception are scenarios where you hit a lot of enemies at once, spells can be very nice there. Skull Trap, Delayed Blast Fireball, Abi-Dhalzim's Horrid Wilting, Incendiary Cloud are the usual favorites for that.
Another exception is enemies with high physical resistances/defenses, which may require spells to damage. However, often it is best to just remove/lower their defenses instead and still hit them over the head with your weapon afterward.
The single best spell, of course, is Improved Haste. It turns you into a slaughtering machine, and will HUGELY increase your damage output. As you are also a thief, another extremely potent spell is Mislead, as @Artemius_I mentioned. If you keep the image out of harm's way, you will essentially be infinitely invisible, turning EVERY attack into a backstab. Personally I consider this a bug and don't use it, but it's in the game so feel free to chunk things left and right!
A F/M/T is probably more effective overall just wading into melee while buffed with (improved) haste and the like than standing back. Aerie is a better source for your offensive spells, as she'll probably have a higher caster level.
Normally I'd add Mass Invisibility to Aremius_l's list, but with only two people, it's probably easier to just use Improved Invisibility.
Spell Immunity: Divination in conjunction with Improved Invisibility... this will stop enemies detecting you with True Sight and similar, and therefore make you untargetable by spells (you can't direct spells at an invisible character).
Spell Immunity: Abjuration protects you from anti-magic spells such as Breach, Dispel and similar. Use this after setting up your usual protections (Stoneskin, Protection from Magic Energy, Pro from Magic Weapons, etc).
Using the two together, on top of your protective spells will make you a nightmare for enemies to take down.
You will use these spell immunities most: Abjuration (all anti protection magic) Divination (all anti illusion magic)
I dislike using too many spell immunities because of how OP they are but if you really want to do so, you can add these: Evocation (most of the damage spells are evocation. Some are necromancy though, skull trap and Abi-Dalzim come to mind) Conjuration ( Symbol:Stun or Symbol: Panic) Enchantment (all status effect except Symbols)
Comments
What do you think are the most important spells I should give the F/M/T?
I will use Aerie as the nuker and divine buffer.
L1: Armor, Shield (early game), Magic Missile (extra slots)
L2: Mirror Image, Blur, Invisibility, Web, Strength (if your strength < 18)
L3: Melf's Minute Meteors, Slow
L4: Stoneskin, Improved Invisibility, Fire Shield, Minor Globe of Invulnerability (for Web synergy), Polymorph Self (optional, not necessary)
L5: Breach, Shadow Door, Spell Immunity
L6: MISLEAD (holy hell absolutely take this), Improved Haste, Protection from Magical Weapons (have multiples of this memorized at all times)
L7: Mordenkainen's Sword + whatever else you want
L8: Improved Mantle (less useful than PfMW but a slot filler), Power Word: Blind, Simulacrum
L9: Time Stop + whatever
As a rule of thumb, you want self-buffs, spells with short cast times and spells that don't scale with level.
Should I keep them to Aerie then?
The exception are scenarios where you hit a lot of enemies at once, spells can be very nice there. Skull Trap, Delayed Blast Fireball, Abi-Dhalzim's Horrid Wilting, Incendiary Cloud are the usual favorites for that.
Another exception is enemies with high physical resistances/defenses, which may require spells to damage. However, often it is best to just remove/lower their defenses instead and still hit them over the head with your weapon afterward.
The single best spell, of course, is Improved Haste. It turns you into a slaughtering machine, and will HUGELY increase your damage output. As you are also a thief, another extremely potent spell is Mislead, as @Artemius_I mentioned. If you keep the image out of harm's way, you will essentially be infinitely invisible, turning EVERY attack into a backstab. Personally I consider this a bug and don't use it, but it's in the game so feel free to chunk things left and right!
Normally I'd add Mass Invisibility to Aremius_l's list, but with only two people, it's probably easier to just use Improved Invisibility.
@Artemius_I F/M/T does not get level 9 spells unfortunately
And how do I use spell immunity best?
Spell Immunity: Divination in conjunction with Improved Invisibility... this will stop enemies detecting you with True Sight and similar, and therefore make you untargetable by spells (you can't direct spells at an invisible character).
Spell Immunity: Abjuration protects you from anti-magic spells such as Breach, Dispel and similar. Use this after setting up your usual protections (Stoneskin, Protection from Magic Energy, Pro from Magic Weapons, etc).
Using the two together, on top of your protective spells will make you a nightmare for enemies to take down.
Abjuration (all anti protection magic)
Divination (all anti illusion magic)
I dislike using too many spell immunities because of how OP they are but if you really want to do so, you can add these:
Evocation (most of the damage spells are evocation. Some are necromancy though, skull trap and Abi-Dalzim come to mind)
Conjuration ( Symbol:Stun or Symbol: Panic)
Enchantment (all status effect except Symbols)