Edit - Heh, I was multitasking, Ryu beat me to the post. Still, I'll leave this here.
I've played the various Flashes in my clan online, plus @
xFriction_Burnx. I primarily zone and use a bit of B23 and F2 to move away or forward, respectively. You'll inevitably end up spending meter to push block his pressure when he gets through your "wall". When I choose to go on the offensive, I utilize B11(stalls, trait cancels/releases, and MB Batarangs when any are possible)to deal with his D1 and hitbox. That and more F2 since it hits mid.
Also keep in mind that they will check you with Lightning Charges from time to time, so punish them accordingly with D2(unless there is something better...no...). Then there's the necessary knowledge of his almost unlimited crossup setups.
It's a tough match in my opinion, but that doesn't mean much. Anyway, that's how I approach the ones I can't auto pilot rush down through. I could be completely off base but I felt like rambling. My performances in this game have been poor thus far.
Maybe it's worth a look to try and find videos of @
DarthArma vs @
Zyphox, @
LEGEND vs Nubcakes, and @
AK Reno_Racks vs @
AK Harold for different approaches and such to help. Assuming those players have recorded footage against one another.