The Divine Link and using the pet to "stop" the doppelganger's regeneration doesn't work (the pet does not stop the health regeneration animation, despite what people keep saying), and I even tried dolls to take advantage of the "attacking stopping health regeneration" which also didn't work.
The instant regen of the doppelganger is game breaking. Even if I final hit with focued beam sabers in Champion mode it's still not enough to overcome that regen.
Chain Crush doesn't stun the Doppelganger. Or rather, the stun indication thing will appear above its head, but it'll still act normally. I found that the skill animation for Chain Crush took a second or so longer to settle compared to Chain Impale during that fight, so I opted for Chain Impale instead. Here is what I did to beat the Doppelganger: I catered 10x Super Sticky Mochi gained from ...
Hit the doppelganger continuously to overcome its health regeneration? I can't even manage to get the first hit to land because the doppelganger seems to teleport straight on top of me before I can reach it. If I try to charge it, my character can freeze for half a second partway through the rush, giving the doppelganger time to teleport to me.
Doppelganger - Auto regen actually stalls when it gets hit by anything - in order to have an easier time with this fight try to have and do the following: -Mana Shield (or really high defense) - a pet (your free thoroughbred works) - Rank 1 Divine link (Preferable)
G12 and G11 did not make me waste tons of Soul Stones, and Doppelganger could be mitigated with a Royal Alchemists. It is also a bit of me doing the fight when it was just released, in the days where Wiki is heavily outdated.