I have a level 19 mage, and my most used spell is definetly fire rain. I have mostly +int and + mana items (archmage staff, ect) and married the frog princess (+3 int). The spell does like 2500 fire damage in a very large radius, and with level 3 skill that allows you to cast spells twice a round for 3 rounds, I can lay down massive damage in the first 3 rounds. Most armies never even get a single unit to me. It would be even better, but I think I got shafted on the attribute selections in between leveling. I think I got offered +int like twice...kinda wierd for a mage. I do have like 20 defense though
Also, a good tactice is to hire a very fast unit with good initiative. The canoneers are a good example. With a damage focused mage it's extremely important to get the first shot. I usually kill anything that can hit me from a distance first, then concentrate on any melee types.
Ice Snake is excellent until you find fire rain...it has a higher damage to the center monster, but the aoe damage is much weaker. It also has a smaller radius and forces you to cast on a target, instead of anywhere. It's good for things like killing those goblin towers though.
Fireball is also good early on, but is just too weak later in the game.
Fire arrow is very nice considering it's mana cost...though I only use it after I have casted fire rain 6 times (2 per round = 120 mana ouch!).
Poison skull is really hit or miss...it has a high max damage...but the min damage is pitifully low. It's really like gambling
If you haven't noticed...I pretty much just made a damage type mage. It was kinda hard in the start to mid game, but once you get the skill that allows multiple spells per round, lots of mana, and fire rain you pretty much decimate anything.