Grimmjow stands no chance in any rounds.
R1: Kenpachi was above Yami who is above Grimmjow. 2 hands and oneshot.
R2: Even bigger stomp, Kenpachi can cut through space, can keep up with a character who is way above Grimmjow, got buffs and still got oneshot by Shikai Kenpachi iirc.
In CFYOW, Kenpachi is the strongest shinigami in the Gotei 13