The trouble is that that AI scrips provided with the game are pretty poor. And to do a decent job of one takes an age - a mage is a complex character with a lot of possibilities, and the scripting language is fairly pathetic. (I get angry about this, they use LUA for the debug console, and their own scripting language lacking even basic loops for the actual game scripting. Makes no sense.)
Anyhow, you will really get a lot of benefit from manual casting - the autopause options mean it isnt all that hard to do, just time consuming. Or you can download a better script,
http://www.sorcerers.net/Games/BG2/index_editors1.htm has a few, probably better than the defaults. Google will turn up a few more as well.
Finaly, you could write your own, but to do that well is quite difficult, not only do you have to be able to program to some degree, you have to know what situations different spells should be used in, and the crudity of the language means that to get anything sensible, you have to do a lot of work. Any berk can write a script to cast your most powerful spells in order, but to get something that holds back on the weaker creatures that dont warrant a spell, trys not to cast spells on things that are immune to it, casts breaching spells before damage spells, keeps a sensible summon around if possible, or any of the other features you might want in a mage, takes effort.