Hello everyone!

Baldur's Gate has Lower Resist, but I don't remember that spell being in Player's Handbook. How did P&P wizards deal with magic resistance?

It works a bit like armour class. The caster must beat a 1d20+caster level check against the target's spell resistance. No effect if the check fails, and the target also still gets a saving throw. Note that innate abilities are not subject to the check, only spells and spell-like abilities.

That spell appears in (2e) Tome of Magic, along with such things as the Numbers sphere and the Wild Mage class. Note that the p&p version of Lower Resistance works the way Icewind Dale's description describes (Magic Resistance half effective, 30% + 1%/level).

Also, 2e Magic Resistance is a flat percentage; the caster's level has nothing to do with it.

If you're using JUST the player's handbook, there's not a lot a mage can do to counter magic-resistant enemies other than to throw dozens of spells at them and hope, or to use summons or other allies. There's also the beatdown method, via shapeshifting/polymorphing/Tenser's Transformation, but with poor THAC0, hp, and attacks per round, it's usually a losing proposition. If you're using more than just the core books, there are a few more methods. Pierce Magic works much like it does in BGII, but Pierce Shield works differently. Pierce Shield allows the next spell cast to completely ignore ALL resistances and immunities, and the subject gets a -5 to its saving throw. (It's also 9th level.) There are also a few rare spells that ignore magic resistance innately, but these are generally high-level.

For the most part, though, there's very little they can do. Magic resistance is a pretty hard counter to 2e mages. Normally you'd have to get creative with your spells and not attack the creature directly, but attempt to cause structural damage or somesuch.

EDIT: Lower Resistance was in later printings/books changed to be 16% + 1%/level, presumably because the original form was too strong for its level. But to my knowledge there aren't really any other spells that work like Lower Resistance, so it seems like a somewhat unnecessary change.