I'm sure after a while Jack will be able to mow the cracker clones down with brute force.
Swap out Crack for Jack vs Luffy & Nami in WCI and they're losing because Jack can outlast them.
Plus with Jack's crazy endurance + cracker only needing to get hit once, I'm positive Jack will get a clean hit. Cracker won't stay back forever, he'll charge in himself to attack like he did with luffy