Rapidly tap Triangle and the X buttons. This can be acheived by rotating the controller approximately 75 degrees colckwise, and alternating hitting the two buttons with your index and middle finger.

You aren't supposed to withstand it, as this is a lesson in attack commands that change, Attack > Reflect, as well as Reaction Commands that keep Riku alive > Reflect.

Looks like someone forgot the "English reading" section of their brain outside of Kingdom Hearts.