Have one or two of your CW or GWF stick with your cleric and protect it from the ads. The fight goes a little slower without more DPS focus on the dragon, but is much safer if you focus on keeping the add count down and keep the cleric alive. All it takes is a couple of imps taking combat advantage on your cleric to drop…
If you find the manual entrance for the dungeon you can still enter at any level. (Entering the Lair of the Mad Dragon through the door east of the ranger camp in helm's hold, for example). You can't qeue for it, so if you need a team to complete it, you will need to bring your own.