Melchiah and Dumah are easy when you figure out what action you must do to defeat them, until then you gotta run. Is not a big deal with Melchiah because you run in circles but with Dumah you have to run across the entire stronghold with a vampire twice Raziel's size behind you, you can't see him most of the time plus he can paralize you :x
It took me a long time to defeat him, mostly because at first I was focused in running away from him ha ha
Later I found out I had to ignite him but is not always so easy to lure and keep him there