The major thing is magic resistance. The easiest way to beat a dragon is buff yourself to the max. Summon some pets to help distract the dragon. Have the spell lower magic resistance and then have your mages pelt the dragon with magic missiles. Because the missiles are entirely magical they cannot be resisted. Use all the ability's you have such as berserk etc... Haste is quite important too. The main damage dealers will be your mages though so keep them safe. Kit yourself up to resist the elements and dont be afraid to use lots of potions.

Well TBH the D'Arnese keep is one of the first quest's I do because the loot and exp is pretty good. Before I attempt it though, I recruit my party consisting of Jan, Aerie, Keldorn, Minsc and Anomen. I aim for 3 utility chars, and 3 melee chars. The black dragon I take on straight away, the red one however I would leave till you have done alot of the other quests. You will find +res/dam dragon equipment through quests and in the Firagg dungeon there are items in the well to help you.

Just make sure you lower magic resist a few times and pelt it with magic missile. Use potions and activate all the beserker stuff etc. You might loose one member but just resurect him and you'll be fine.