A common strategy involves a knight blocking while several Paladins and/or Mages stay as far away as possible from the creature while shooting bolts and or runes. Mages can also summon demon skeletons to block the creature as they are not affected by fire. Knights when blocking are advised to stand diagonally to the dragon lord to avoid the fire wave attack; a practice often called dancing with the dragon; although it is no display of affection.
If you are on a non-pvp world, a Knight and a Druid can hunt like this in big spawns (such as Pits of Inferno Dragon Lair):
The knight lures a few DLs, and keeps standing at one spot. The Druid has to stand a certain number of squares away from him, and, using Ice Wave, hit all the DLs that are attacking the knight, also healing the knight with exura sio. LOTS of experience can be obtained like this in a short time.
Higher level players can solo dragon lords in different ways as set out below. Knights 60+ can use melee and Exori-spells, healing as required. But it is hard to profit since a great number of Mana Potions would be used. Mages (recommended Level 45+) can use attacks like Ice Strike or Icicles to kill dragon lords, supported by some Avalanches when they are trying to run away, since they're weak against this element. Alternatively they can summon Demon Skeletons to keep the dragon lord at a distance and shoot runes. Keep in mind that the dragon lord will kill the Demon Skeletons quite fast, so you should be bringing a few healing runes, alternatively summon a new one. For mages below Level 60, it is recommended to use Magic Shield and always stand diagonal.
Paladins (recommended level 55+) can shoot bolts or Ethereal Spears while running to avoid the fire waves and melee attacks.