Basically, Dark Magic is biased towards combat, and Light towards protection. If you want combat spells, the two best IMO are Shrapmetal and Dragon Breath. You want to get your cleric and sorcerer up to Masters in Dark Magic with a skill level of 25 or 26 (but no more!), and equip each of them with a Ring of Dark Magic and the artifact ring Guinevere. These two interact to boost your effective skill in Dark Magic by 2.5, so your skill level of 26 becomes an effective skill level of 58. Under no circumstances must you go over an effective level of 60 - it's more than the game engine can cope with, and your performance drops sharply.
There's a particular technique to using Shrapmetal to get the most out of it. What you do is rush up to your selected target until you can get no closer - you're at contact range. Then cast the spell. You might find this easier to handle if you switch to turn-based mode once you close right in. A gold dragon will go from undamaged to dead in no more than 3 casts of Shrapmetal by Dark Magic masters developed as outlined above and using this technique.
On the other hand, if you want to do massive damage in hand to hand combat, your best weapon is probably the relic spear Poseidon, used alone, (get your skill level in Spear well up) and boost it using your cleric's spirit spells Bless and Heroism to boost performance. In this case, you will need to give your cleric a Ring of Spirit Magic and the artifact ring Igraine. Again, don't get the base skill level above 26. Igraine will also boost the other self magic spells - Mind and Body Magics.
You'll probably have gathered that there will be an artifact that boosts all the elemental magic spells (Fire, Air, Water & Earth) as well. It's the amulet Morgan.
You should have the general idea from the above, and you can pick pretty well however you want to play it. Get back to us if you have other questions.
HTH! 
