Assuming you're after single kills at a time...Originally Posted by Allen
Take a good look about before a pull, if there's a lot of mobs start clearing from a safe point. Mid-fight adds are much worse for pure casters in general than tanks or pet classes because we need range to be effective. Also take note of which way to run if you need to.
When pulling throw as much damage out as possible using your initial range. This means standing right on the edge of fireball or frostbolt range (both of those are better in general than arcane missiles for first strike) and just casting as quick as possible till they reach you.
Once the mob reaches you I favour the fire blast, frost nova, back off, nuke routine. You take less damage than trying to duke it out toe-to-toe and your interruptible nukes are more efficient. If your root fails then instants and arcane missiles are best (if you have improved missiles, otherwise, scorch is a good choice due to speed). Don't be afraid to put up your mana shield if you need it, or just plain run (blink can help to get range quickly) - it's better to run and live than die and have to res up.
Having weapon skills at a reasonable level is also useful for finishing that mob off on 1% - I currently carry a sword and it can hit for about 1-1.5% of a mobs HP per swing.
If you have the right talent (imp. arcane explosion) then multiple mob pulls are fairly easy. You'll have downtime most pulls as it's fairly mana intensive, and often HP intensive and it only works on tank mobs really.
Basic strategy is to pull multiple mobs (3-4 at a couple of levels below is about a nice number), either run through or nuke them - fireball/scorch, scorch, fire blast is a good way to pull 3 mobs for instance. Then let them run to you and frost nova, take a couple of steps back so you're just out of melee. Hit cone of cold (particularly if ice specced), then spam arcane explosion till they die. It's doable but tough without improved arcane explosion, with it's pretty easy.
I'd certainly recommend every mage get 16 arcane for arcane concentration (5/5), improved arcane explosion (5/5) and evocation. Those 3 talents provide a lot of utilty and power for the mage class. In most high-level instances (50+) you'll be pretty much expected to have imp. arcane explosion to AE mobs with, but I wouldn't worry too much about rushing to get it before that.
Lot of text, hope that lot helps you a bit.