Doppelganger

Is it possible to kill the Doppelganger while it is in Demigod form? I recall that you can damage it, but its super fast regen makes it look like you did no damage at all.

Prrrrobably not. You need to look at it in the terms of stats. The Doppelganger is severely weakened in stats in the second stage, but is on par with the Elf/Giant guards... injected with Chuck Norris in the first stage. Theoretically speaking,, you should be able to kill it in the first stage. Then again, theoretically speaking, you can kill anything in the game, because there's no such thing as the infinity value in programming, just values close to.

I have watched my sister attempt to kill the G10 final Doppelganger during monster lag, in stage 1. She got the health down to 0 but the Doppelganger did not die, meaning she was stuck on Stage 1 until the Music Box and Life Drain were used against it.

Did she even attack it after its health was down? o-o

She did. She was in full deadly and she kept hitting the Doppelganger. The health of the Doppelganger is similar to how Mana Shield doesn't stop when your MP goes down to 0 in Demigod.