Instead of blocking, try moves that have i-frames. Ninja can use free cancelable jump kicks to close a short distance and be immune/actionable. Cowboy can dash and use instant teleport. Wizard probably has the least options here, but magic darts can force the player to do something else.
Viewing post in Cowboy draw/holster combo + temporal rounds is OP
When you try using jump kick or roll you evade incoming bullets but at the end you are open for another shot, so that's not a really viable option unless you are so close to hit the cowboy. And even if you use moves for free it just means you lost your cancels instead of health and are in a worse situation than your opponent. Looks to me the only viable way is to stay at close range and prevent cowboy from using this setup. And even then is almost impossible as he can just teleport up in the sky, build meter and shower you in a rain of bullets.