Xeta

Because he's a boss. -_-

Go into Practice and learn some combos for the speicfied Character you wish to play as.

cheating-is-good

I know, Jinpachi Mishima is surprisingly hard, even when you set the game's difficulty level on easy. The most trouble I had with him was that stupid fire ball that he shoots out of his stomach. If you are having trouble with that as well then I will tell you that ducking is useless, you actually have to side step to the left, or sidestep to the right in order to dodge it. But watch out, because sometimes he will launch out a second fireball when you least expect it, so be on your guard. An easy way to beat any of the characters or bosses in the game, is to crouch down, and just keep pressing either the X button, or the Circle button repeatedly so that your character keeps kicking Jinpachi in the shin. This is a little unfair, and could be considered cheating...but hey, everyone has their methods right?

Alternatively, you could go to practice mode and learn a bunch of combos with your character, or do the all time method of randomly mashing buttons.

Hope this helps. :)