To perform a brave edge you must use two stocks of your Critical Gauge to perform it. Brave Edge attacks ca only be performed after using certain normals, a successful brave edge attack will glow a yellow trial and if it hits will leave a yellow static on the opponent. All brave edge attacks are done by pressing ++ after specific inputs. You can view the possible brave edge attacks in the character move list in the menu under Edge Attacks.
If you do not feel up to grinding this out in any of the offline modes, you can boost this with a partner online. If you are having trouble finding a partner, please refer to the Trophy Boosting Thread.