This worked for me, don't know about you

I'm a R/Mo and I struggled to beat the doppelganger with a number of different skill combinations.

I tried doing it with my attributes split evenly between marksmanship and healing prayers, and removed all skills except healing breeze, and did it with about 90% health left. My doppelganger didnt use it once, and just kept attacking me.

Not sure if it works for everyone, but it did for me.

It's incredibly easy to beat the Doppleganger as a Ranger, take 2 skills:

Distracting Shot & Troll Ungent

Then run up to him, he'll pull out an axe. Since Distracting Shot is a bow-only skill, he can't use it on you with his axe out, but you can use it on him when he tries to heal. You can heal, he can't. You win.

Or just bring every spirit you have...and penetrating attack

beat it in under a minute like that.

16 marks/expertise is all you need.

I used only Pet Skills and beat him in 20 seconds flat.

Apply Poison, Dist Shot, Any spirits you got.

Apply before you go on the last stone and enter the fight, run up close and just shoot him. Distract his apply poison.

Dumb dopple dont use healing breeze...ever....its a bug....he also doesnt have a pet.
those r easy ways to beat him

I think it uses Breeze purely to counter degen. Not sure if thats true though, i prefer to just kick its ass.

Nothing beats waiting for its Fertile Season to wear off just so i can laugh as we both drop dead at the same time. Not sure why though...