Stratagies!

 
 
When your at level 20 or so if you see a whole swarm of demons use gardian, it rocks! Then run away and watch the s.o.b.'s die. And if there is still some guys left over use lightning to kill them (because it goes through guys). And don't foget when you level up even it out so your warrior won't suck.
Play like a rogue. Shoot till they get close and then run away till you can turn around and shoot again. Don't engage in hand to hand. I almost never engage in hand to hand after level 10. When you run away be sure to run to where you have already cleared the monsters from, cuz you dont want to have more chasing you. If you have to, run around something like a stair well or something. I was able to kill 8 Blood Knights like that.
Use Mana Shield. Use Mana Shield! Use Mana Shield!! Learn the spell or buy the book as soon as you can. Then spend your level points on magic as you get alot more mana per magic point then you get life for vitality points. You will have more hitpoints then everyone else. 
Watch out for your teammates! Dont use Fireball when you are shooting near your teammate as it will hurt them too. Same with Lightning. 
A great tactic for getting fast XP as a sorceror... When you have just reached lvl 13, either get the book of golem or a couple of scrolls and go into the caves. Making sure that there is no monsters nearby to kill you, and cast a golem; the golem should be able to clear out a large proportion of the level, given time, and a few should almost certainly take out most of the monsters. Then you can run around grabbing some nice items, and you should also have gained a nice amount of XP. 
When you're up against something that is quite tough to kill (like bosses in the catacombs playing in nightmare mode) is to stock up on mane, and cast golem. While the golem is fighting the creature (bosses, large groups of enemies, etc.) keep stone cursing the creature(s) under attack by your golem. The golem doesn't get a scratch, you don't get anywhere near the boss (ie - not getting hurt), and you get a nice magic item for the couple of mana potions you spend on the stone curses.
Gaining XP at very high levels:
A friend and mine an I are both 35th level mages and are battling in hell mode hell now. It's about the only place that still gives us exp points. Down there the monsters are pretty much immue to everything so we figured out how to do it. The secret is to go down "naked." You're going to die 5 or 6 times a level and you dont want to lose your stuff! If fireball and chain light dont work then use stone and hit them. My favorite is to buy a apoc staff and nuke them from behind walls!! Then TP and recharge. Once you're on level 16 you get your stuff back when you die so then just put it on. I discovered the "naked" trick when I lost evething at the entrance to hell and couldnt get it back so I went naked through all the caves to finally come down the stairs from level 12 to 13!! Mages are strong at high levels and you really dont need any of your stuff except the resistance items and lots of mana.
 
Top 10 tips for magi:
1) Use a bow. Staves are an expensive pain. I use the ones I find as disposalbe weapons. staves of healing should be stashed near a direct to town stairway.

2) With multiplayers practice teamwork drills on the upper levels. Proper Previous PLanning Prevents Piss Poor Performance. Warriors have all too often gotten in the way of ranged weapon support.

3) Work out simple emergency code letters (that can be typed with one hand).

4) Decide what directional references you are going to use. We use the 'clock' method.

5) Use the terrain and blindspots as cover and for ambushes.

6) Cache weapons and other goodies near stairwells on floors that have been cleared.

7) Even high level characters can shop for books on the 'easy' levels. Sometimes the book is what you can use, sometimes not.

8) The Windfall Engine cuts down on a lot of walking. Being able to buy all the books Witchie Poo has is a big help. Having lots of scrolls helps until you get the spell.

9) With a pack full of Potions of Mana and Healing I don't carry a bow anymore. I use the same hit and run tacits with the Firebolt spell. <which until recently cost a lot of mana>. I do a manual Flamewave against packs of archers.

10) The Lightning spell works great in chokepoints and along walls where the baddies line up.
 

One of the most simple strategies to use with the sorceror is to put all your 'level' points on magic, you will eventually find a book of mana shield, cast this and your life should effectively be up around 250!
A great way to save your self from unexpected monster attacks before you round a corner or go through a doorway is to get out the charged bolt spell and point it in the general direction of the corner/doorway. Fire off two or three bolts (the cost is relatively small) and then listen carefully for the "grunts" from the monsters. If you hear any, simply fire off a few more bolts until the monsters stop making noise. The bouncing effect of these bolts make this an easy way for the sorceror to kill monsters around corners sight unseen. One note of caution; if a monster is right against the near side of the wall next to the corner, they probably will not be hit by a bolt, but hey, what's life without a few risks. This tip is really usefull on the lower levels when mages have no "real" life points and should try to avoid head on combat situations.