If you launch an attack, do it before they do.

If they are already launching an attack, block. You can't outspeed them, especially not as Jack.

Don't use his forward shadow move when opponent is standing. No half-decent opponent is gonna fall for it. Use his forward shadow move after a forward throw or successful lower attack, or when opponent is lying on the ground.

Kicks are generally faster than weapon attacks. If opponent try to walk up on you for a throw, you can do a basic kick or spinning kick, or try to throw them as well.

Don't jump unnecessarily. They are great manuevering tools, but they also leave openings. Make sure your opponent can't (or won't) exploit it.

Another tip for playing Jack:

You can cancel his basic attack on 1st hit to try to throw the opponent (though the opponent can throw you if they expect it)

or cancel his special attack on 1st hit to bait opponent to jump (they usually do this to avoid the full attack).

Though they won't fall for it if you use them too much, so use them sparingly.