I had a similar issue. I gave him a couple backup clerics, one that had cast Shield Other on Kharn to reduce the damage he took. Round one the party Artificer//Warblade (we play Gestalt) charged him and dealt 200 points of damage. Enough to kill Kharn and the unnamed minion in a single hit. You know what I did? I went with it. He got off a spectacular attack and slew him in one hit. So the following round when he used Intimidate to scare the stunned Hobgobs, I gave him a +10 circumstance bonus. He's now known across Khorvaire as the guy who is so good with a blade that he can kill two people with a single strike. The point is don't be afraid to let the BBEG just die in the first round. It can lead to great stories in the future. Give Kharn some standard buffs ahead of time and maybe have a backup Cleric cast wind wall to protect them from the stream of deadly arrows and see how things turn out.
My players are just now getting around to dealing with the Ghostlord, and we finished the RHoD months ago. I made him a Planar Shepherd (linked to Mabar, the Endless Night) and gave him Corrupted Wild Shape feat. He's a major threat to the safety of everyone in the area due to being crazy and having ambitions to unite Mabar with Eberron.
A white dragon would be pretty cool to use. I would plunge the land into an endless winter and use tons of undead. They won't be bothered by the cold and can move about just fine, killing and destroying much needed winter supplies left and right. Starvation can indirectly become a threat to the PC's at a level where it's usually ignored. And white dragons get a really bad rep they don't deserve. They may have the lowest mental ability scores but they are way more intelligent than most other types of creatures. Finding a way to control the weather is easy, just say it got a Fimbulwinter scroll and succeeded on the UMD/CL check.