I have only killed the Doppleganger twice. Once with my W/me and once with my Me/E. Both times I killed him in about 20-30 seconds. All you have to do is keep in mind that the Doppleganger has everything you have (plus maybe a bow), except....that it is AI. Just use that fact to your advantage.
With both characters I killed him using the skill set I had equiped for the last mission.
With my mesmer I let him use interupts on me on some fairrly low cost/dmg spells and then interupted his high cost/dmg spells. For example I let him interupt conjure phantasm and fireball, let him hit me with those, interupted his meteor and meteor shower, and hit him with the same...the fight was over in just over 20 seconds.
With my warrior I used a similar tactic. Interupted his Dwarven battle stance and let him interupt my heal sig (when I didn't need it) and then hit him with DBS and my other dmg dealing skills.
Use the same tactics when you fight it. If you bring empathy don't use physical attacks against him and he'l kill himself. If you bring smites, be prepared to counter them...
hope that helps
EDIT:
yes there is a bonus for time, but I don't remeber exactly what it is. Maybe if you kill him in less than 40 or 50 seconds you get it. It will tell you when you get there if you want to take time to read it.
