These little bastards appear at level 14 as Baby power dragons, and evolve up to Ancient Wyrms of Power. The trouble here is that they breathe every element in the book: acid, elec, fire, cold, poison, nether, light, dark, sound, chaos, disen, nexus, time, inertia, gravity, shards, plasma, force, mana and water. The main consequence is that they also resist every element in the book, making most of the spells against them totally inefficient: they will resist (half damage), greatly resist (1/9 damage)... or even be completely immune.
The best way to deal with them is melee or missiles, if you're a melee fighter or an archer. But with spellcasters, you may be in trouble. If you want, you can rely on magic devices: a wand of annihilation will do 250 damage per charge, a staff of holiness/power will do 120 damage per charge; other devices like wands/rods of drain life or staves of dispel evil can also be used, but they will do much less damage (60-90). Of course, you'll have to recharge the devices, and they can be drained or destroyed...
Let's see what spells could be used with each class:
Mage:
You have two unresistable spells: Magic Missile and Meteor Swarm. Magic Missile is available at level 1 and will do 3d4 (average 7.5 damage) at level 1 to 12d4 (average 30 damage) at level 50. Meteor Swarm is available at level 30 and will do 135 damage at level 30 to 220 damage at level 50.
Stategy: use Magic Missile at low level and Meteor Swarm when available.
Priest:
Here you have multiple options. Orb of Draining is available at level 9 and will do 13+3d6 doubled against evil (average 47) at level 9 to 75+3d6 doubled against evil (average 171) at level 50. Dispel Evil is available at level 25 and will do 1d75 (average 38) at level 25 to 1d150 (average 75.5) at level 50. The improved version is available at level 20 (if you have the book that is) and will do 1d80 (average 40.5) at level 20 to 1d200 (average 101.5) at level 50. Annihilation is available at level 45 and will do 200 damage.
Strategy: use Orb of Draining (or simply use a good Mace of Disruption and healing spells).
Sorceror:
Like Mage, you have two unresistable spells: Magic Missile and Annihilation Bolt. Magic Missile is available at level 1 and will do 3d4 (average 7.5 damage) at level 1 to 12d9 (average 60 damage) at level 50. Annihilation Bolt is available at level 39 and will do 478 damage at level 39 to 500 damage at level 50.
Stategy: use Magic Missile at low level and Annihilation Bolt when available.
Telepath:
The only unresistable powers you get are the psionic ones (the kinetic ones are force-based). Mind Blast is available at level 1 and will do 3d3 (average 6 damage) at level 1 to 15d6 (average 52.5 damage) at level 50. Psychic Drain is available at level 25 and will do 12d6 (average 42 damage) at level 25 to 25d6 (average 87.5 damage) at level 50. Mind Wave is available at level 30 and will do 45 damage at level 30 to 75 damage at level 50. Brain Smash is available at level 48 and will do 28d15 (average 224) damage at level 48 to 30d15 (average 240 damage) at level 50.
Strategy: use Mind Blast at low level and Brain Smash at high level (you can throw a Psychic Drain here and there to replenish your mana, or even use melee with Adrenaline Channeling when you reach level 50).
Elementalist:
As the class suggests it, you only get elemental powers.
Strategy: use melee (hard with only 3 bpr), missiles or devices... no other choice here.
Summoner:
This class doesn't get a lot of offensive spells, and they're all elemental.
Strategy: let your summons do the killing.
Necromancer:
This class is quite versatile for a mage-like spellcaster, though you won't get either Magic Missile or Meteor Swarm. Drain Life is available at level 30 and will do 100 damage. Cloud of Draining is available at level 40 and will do 100 damage.
Strategy: like Mage, you can use melee (4 bpr), but once you get Shadow Touch and Absorb Soul (and later on Touch of Death) you'll get a serious advantage as your melee blows and kills will heal you (and even score mighty critical hits); you can also use corpses to raise undead slaves and let them do the killing.