I find that, until you're ready for this battle (ie, while you're still in Seyda Neen), you can avoid the attacks by sleeping in the keg cellar of the Census and Excise office. I've gone round-n-round with other forum members before on this issue, but I've never (in hundreds of times sleeping there) gotten attacked by DB in that cellar bedroll.
[edit: spelling]
This is interesting. Wouldn't you think it's all random, though?
I still think that particular DB assassin attacking you IS a joke. He makes loud noices while moving (apparently breaks something on his way to you... so cliché!) and goes down like an elite assassin should not go.