Oh, you'll definitely realize how good that one move would have been in that situation one second after it's too late.
Hi, there, Cynical, welcome to the world of Mitsurugi. First thing I recommend is when using 66BB, learn to delay the second hit and throw it out only when the first part hits and launches. It's probably the easiest hit confirm in the game next to one of Z.W.E.I.'s Brave Edges. If you throw the second hit out while they're blocking, you're going to get jabbed in the face.
Don't worry about being bad at the game even after sinking time into the series; I've played all six games and I still don't guard impact. *head hangs in shame*