There's a few ways I like dealing with mages, though I'm not entirely sure if you would consider these methods dependent on meta knowledge:

The Inquisitor kit is really good at rendering mages helpless. True Sight gets rid of several annoying defensive effects, like Mirror Image, and the Inquisitor's dispel casts at twice its level and helps get rid of everything else (I think friendly fire is an issue here though so be careful with it).

Thieves in general can do a lot of damage to mages with their Backstab, often insta-killing mages who typically have low health. Applying the Assassin's poison ability to a ranged weapon can also have devastating effects, the poison ticks interrupt most casting. The same applies to anyone wielding the Dagger of Venom.

Another option is to stay just out of sight and chuck Skull Traps or fireballs into the fog of war. NPCs will catch on pretty quick and come after you, but most mages don't have enough health to last that long.

In the event that they do get spells off it's not always a total loss. I usually keep a Remove Fear spell handy (this is useless if the caster that has it gets feared though), and the aforementioned Dispel Magic can remove detrimental effects from allies, though I think a die roll is involved, making it somewhat unreliable.

In BG1 mages can definitely be among the more challenging opponents though, so you might have to get creative.