To be fair, Jack's also trained since he was a child, and not just physically either. Plus he's had over five decades of experience.

And the girls were hardly mentally sound themselves.

Not to mention jack might just be more than immortal at this point, like you don't survive getting stabbed in the gut and walk that off a couple of days later. I know I've seen a few people theorizing that not only has jack become immortal but invincible (ie immortals don't die of age, invincibles can't be killed) but I think it's more likely that he's not invincible but just has some supernatural regeneration due to his apparent immortality.

Either way if this series doesn't end with jack going back in time to become an immortal god king I'll eat my shoe.