Probably like everything else..by pressing LT and Left Stick :)
Counter he wuld use the Wind Ninpo.
What would that do? I've never played a Ninja Gaiden game. I'm not assuming Kratos would win or lose (nor do I really care) so I'm just curious as to what Ryu could do against an attack like Poseidon's Rage.
As of the end of God of War 2 (thanks to Urns), Kratos has infinite magic so he could keep it up forever and can never be interrupted or knocked out of his animations.