I was wondering, if anyone knew how I have to kill Operator IX for it to count towards the achievement. I killed him several times in story mode, which seems to be the right way to kill him. I killed him with several characters, both Imp and Rep.

But... I only killed him in 8-man-version and I'm not 100% sure, if I killed him with all 4 colors available. It might be I'm missing one. Does anyone know if you actually have to kill him in 16-man-version to count for the achievement.

I know it's only him missing, I checked my progress.

Thanks in advance.