Gloom Razor only works if you hit a flanked opponent, so you need two successful attacks on round 1. The first attack causes your opponent to be flanked, the second attack removes the flanking but activates Gloom Razor. So you'd probably want Snap Kick to get two attacks on a standard action.

Hmm. Seems like it's too easy to shut down with a 5' step, once your opponent is no longer adjacent it stops working. There's some maneuvers that prevent an opponent from moving. Or maybe some Knight levels + reach weapon? You can't 5' step in difficult terrain.