Well, I managed to snag a game with Dclone spawned, and my assassin took at him again. I say I lost because I died five times. On the other hand, I have learned plenty and I plan to put it to use for my next encounter with him.
First: Hotkey my damn life tap wand!
Second: Some more Fire Resistance.
Third: Do not hesitate to break off sooner.
Fourth: Dragon Flight is handy for evading his Firestorm.
And I know my gear could be better, my point was to use trash and succeed. :)