Are there shortcuts for EX Burst in Dissidia Duodecim?

Answer 1

R + square, also you can equip the Auto-EX ability, your character will auto ex-burst once the EX bar is full =)

Answer 2

Here's a video with all the methods for EX Burst if you're intersted:

http://www.youtube.com/watch?v=g0X23y35lIA