This seems daunting, bringing thoughts of mastering manual linkers and such, but is actually extremely easy with Omen. Just start up a versus match against a 2nd controller. Have the 2nd controller block while you do basic quick punches and kicks to get your 3 shadow meter bars full. Stop blocking with player 2 and unleash his Demonic Despair move (down, down forward, forward + quickpunch and quick kick). With default controls this move is
This causes the dummy controller's entire life bar to become potential damage. Then do any basic combo completed with a finishing move to turn all that potential damage into actual damage for a combo of at least 85% damage and you'll unlock the achievement. I'm working on a video guide for all of Omen's achievements. Thank god for no 150 fight titles one eh?
This one is at 02:30 in the video below.