The Achilles heel for non-player spellcasters is mana. For player spellcasters, it tends to be health, but enemy spellcasters can often be really strong, particularly dungeon bosses. However, they still have stats like the player, and that includes a mana stat. They start a battle with full mana and drain it as they fight. They will not regenerate mana at anywhere near the rate they spend it, so if you can get past that initial period of heavy spell-slinging, things get a bit easier. As jht said, if you can do things to reduce their mana, that's a really good idea. Another thing you can do, and one I often use, is to just let them use it up. Their spells have to hit you just like archers' arrows do. Keep enough distance from them and you can dodge the spells. After a bit of dodging, the spellcaster will get to the bottom of their mana pool and their spells will come less frequently. Then charge in and whack away at them. Having a bow and arrow to do some damage while you're waiting for them to run dry will help as well.
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