My qualifications for this are that I have soloed more than 300 dragons. First off one needs to know how they hit and where they are weakest. They weakest against poison, then cold, and then energy. Their attacks are 100% physical except for the flame attack which is 100% fire. It is more important to have a high fire resist when fighting them because you sometimes will be hit more than once by a fire attack.
Be sure to have a reasonably high magic resist or you'll spend a lot of time healing poison.
Need I say that a good Dragon slaying weapon is necessary?
The easiest way to kill them is with an archer or mage (I won't discuss bards or tamers because I don't play either class and my advice would be less than worthless).
With an archer target the creature and then move out of bow range. Wait. As the dragon approaches, your shot will go off as soon as he closes to range. Immediately back out of range and repeat.
The spells cast by a dragon can be tricky early in a fight, especially if you get hit by the breath and then suffer a combo like flame strike/ explosion. Don't hang around after you suffer damage but get out of range immediately, disarm and drink a heal before that explosion goes off and finishes you.
After the dragon has depleted his initial mana pool, the danger drops because he will use up his mana as it recovers in casting low level spells rather than waiting to accumulate mana for a larger spell. In fact giving him a target for the low level spells is a good way to keep that mana from building too high.
Few people realise that poisoning works great on dragons as long as you use poison fields. The reason is that his body is so large that he will be poisoned three or four times while passing through the field and will keep spending mana casting cures rather than cooking you.
Melee works best if you have a long range hitter helping you. Wait until he's focused on the archer/mage and the come up right at the base of his tail and swing. Dragons pivot in two stages of 90 degrees so you can hit him and disengage before he can pivot on you. Don't dawdle. Get in and get out. Most people get killed because they stick around after they get hit the first time from some misplaced sense of macho honor (testosterone poisoning I call it).
When in a group, the archers should be moving between shots because this movement makes the dragon retarget to you and you want the dragon targeting the archers not the melee fighters. Only later when the dragons hit points fall below 25% do you want to stand still and pour the fire in to finish him.
If you are a melee fighter and you are on the scene alone before help arrives, don't engage him. Just step out of range and circle him to keep him in one spot so your friends can find you to back you up. A melee fighter can take a dragon alone, but why bother when you have backup?
When hunting in a group, the rabbit has to remember that her job is to lure the dragon back to the party, not to kill it.
With a little humility and commonsense, ther is no reason for anyone to die to these big slow critters.