I would have picked the Inquisitor or Cavalier class if I didn't have to be Lawful Good (which is just boring).
Just a note here, but your protagonist's alignment has almost no impact on the game. It will affect your starting reputation by a couple of points, and if you're evil, turn undead will control the undead instead of fearing and/or killing them. It'll also affect your starting innate abilities, but those aren't exactly gamebreaking, and all of them are useful. A protagonist with an evil alignment also gets a couple of quest options that the others don't get, but those options are so silly and mustache-twirlingly bad that (from an RP perspective) you'd have to be evil AND stupid to choose them.
Your alignment isn't a straightjacket in the BG games, unlike some other D&D games. It's just a roleplaying tool. You can play a deviously-evil "Lawful Good" paladin, and if you're devious enough, you can do it with a full 20 reputation to boot.
All that aside, there are a couple of low-level priest spells that can really turn the tide against casters. "Silence 15' Radius" and "Hold Person" can neuter them entirely. You can also use magic missile to eat through all of their mirror images quickly, and weapons with elemental damage on them will bypass Stoneskin and disrupt spells. Against higher-level mages, "Breach," "Pierce Magic," "Spell Thrust," and similar spells will specifically dispel their defensive protections.
High-level D&D Wizards are very very tough, especially in BG2. You have to fight them with very different strategies compared to other monsters. Wizards generally have almost no hitpoints, but lots of tricks to protect them, and powerful offense. Counter their tricks, and they'll die to a quick swording.