Jason_the_Iguana: You don't need breach. Really, it's overrated. People solo BG2 with berserker fighters and the like. I myself have played the first two chapters with a party that only had a bard in it for arcane casters. (Though I did have a cleric.)
There are three kinds of defences mages use. Stoneskin, which can be bashed through by just attacking a lot. Mantle/improved Mantle, which can be bypassed by using a more powerful magic weapon. And Protection from Magic Weapons which can be bypassed... by making sure your fighters have a non-magical weapon. Wizards still die to a normal sword. (Doesn't work against Liches, though)
Then there are area-effect spells (not an option for a fighter party, but potions of firebreath and such can work) and weapons like the Holy Avenger or Arrows of Dispelling that can break defences. Those green protection scrolls can work a charm for the toughest fights, and potions can give your guys enough magic resistance to laugh off anything mages do. (For a paladin, getting 100% resistance is a breeze with potions and the holy avenger.)
Edit: Oh, and you can easily make a trip to Watcher's Keep and buy/loot a bunch of wands that cast breach if you really need them. The first level isn't too tough as long as you do not activate the guardians.
Or you can just tough it out. Enemy mages generally won't kill your party in the four rounds it takes for these spells to wear off. Just spread your guys out so only one or two tough ones get hit by the area effect magics. Then hit them as soon as you hear the noise of their mantle dropping. Typically, all you have to deal with is the odd meteor shower and symbol of fear. Still, might be a good idea to spend the cash you save on scrolls on a nice rod of resurrection in the Adventurer's Mart.
Finally, you can solve the problem pro-actively: have your rogue open up hostilities with a nice 5X damage backstab. You only need to breach defences if the enemy has time to put them up. I loves seeing the enemy's contingency spells firing -after- their body hits the floor.
As for potions, I try to use them quite heavily in both BG and Icewind Dale. Potions in these games are incredibly powerful. Sure, a mage can cast Haste on the whole party. But an oil of speed will last five times as long. A potion of Giant Strength will also last long enough for an entire dungeon level, and is -far- more effective than spells of the same type.
Next time I play through BG2, I'll keep what you said in mind about the mage protections. Carrying a few normal weapons just to counter protection from magical weapons isn't something I considered because in BG2, inventory space is at a premium. It's also one of the reasons I didn't hang on to potions that can be duplicated by spells. When you also carry healing potions, those potion bags and inventory slots fill up quickly. In addition with regards to IWD, I usually needed the inventory space for arrows because in Dragon's Eye when you kill the high archer for Messenger of SSeth, I keep that bow until the end of the game but it just chews though ammo and there are no everlast arrows like in BG2 and IWD2. Compounding the arrow problem, there is usually at least one other bow user also firing away meaning making more space for more arrows a priority.
However, I realize this all comes down to chosen tactics. I might have to do a no ranged weapon playthough sometime and see how potions can compliment magic and melee ;). It's gonna be strange though cos my wizards are always in the back slinging away. For that playthrough, maybe I'll take a fighter / mage or two as my main casters as straight single class mages will just be dogmeat (and I don't want to resort to "rest every second to replenish spells" kinda cheese either). Also, no ranged weapons at the end of TOTLM? That's gonna be hell. :(