So with a level 47 Enervating str/con debuff your debuffing their con for 118 so that would be a nice chunk of damage, 472 or so on an assassin? Is the debuff applied then your damage or you damage then the debuff?

Your dammage then the debuff.
Most people use it for the str debuff.

I think most people use it for both the str debuff and the un-healable, don't-have-to-PA-for 300-500 damage.