Acacia said "you guys (disciples) can't beat the Blue Nitro", not "no one can beat the Blue Nitro.", and don't forget that was the disciples 500yrs in the past. They are exponentially more powerful in the present than they were back then. And finally, Kaka was never stated to be a Blue Nitro.
Yeah, I know. In fact I was comparing Acacia's disciples with 4HKs, but I suppose Ichiryuu, Jirou and Midora were already stronger than the actual 4HKs.