Rather than sending Jack into the future, Jack is instead sent into the world of Berserk post Eclipse. Griffith has been reborn, Guts is slowly building his new band of friends, and Ganishka is still a jerk. Jack deduces that he must kill the 5 Godhand in order to get back.
Can Jack basically find a way to do that or will the 5's command over causality be too much for him?
I think with help from Guts and Skull Knight, he figures a way out. He's at least easily on Guts' level as well, and has a speed advantage. Plus, Jack's biggest advantage is that his sword is basically tailor made to kill every bad guy in Berserkverse.
And if it took the actual combined might of Odin, Ra, and Vishnu, then it's a holy weapon capable of even killing God Hands imo.
Jack would easily kill everyone short of the Godhand.
You remember when Guts and Zodd had to band together to fight Ganishka back?
Jack would have killed him. Easily.
There is not a single being below the level of the Godhand Jack couldn't defeat fairly handily, and even they could be killed by his sword. He just might never be in a position to use it on them due to their power over causality.