50 Combo Achievement

When playing through the game, try aim for a 1000 point combo. This will unlock the Combo Window: LONG Ex Option. What this does is makes your combos last longer, giving you greater time to get the 50x. I found this easiest on the later levels, Stage 5 in particular. Don't forget to use Climax Mode, this really helps towards the achievement as you can knock out many enemies without them knocking you out first.