I can give you some strats if you need them. Or is he just beating you bloodily despite you having a perfectly legitimate gameplan? Because that's what happened to me.
**** Chaos pissed me off... Just keep at it... Once you beat all three rounds you feel that much better...
I would suggest to equip some of the Resin accessories like Arcane Resin, Patience Resin, Destruction Resin, and Valor Resin to increase your attack, defense, bravery, and ex gauge. They give a big boost in stats and they dont break until the the third battle is beaten.
Yeeeeah. Dissidia may not be a proper fighting game, but it sure has a proper final boss. Well, as a rule of thumb, just don't panic when he breaks you; it'll happen a lot, but you know that. Just ride it out and wait for an opening to attack. The more you get used to dodging and blocking his attacks, the closer you'll come to whupping his ass. Your biggest concern through practically every round is Condemn; always have a dodge ready to jump out of the way of it. Everything else is heavily telegraphed or else pretty easy to dodge/block, so just try to get used to the timing. I don't know how to beat him with Zidane specifically, but you do have the advantage of being able to jump straight out of blocking his claw swipe with an HP or a Brave, and you have a good aerial game, which is usually where you want to be with Chaos because that closes a lot of his better attacking options. I don't know if you have any good wall rushes, but try using those a lot; he has trouble with them. You might be able to get him stuck in a wall rush loop, like I did with Terra's Blizzard shot. If you can't, though, he'll probably respond with a close-range attack as soon as his hit stun is over, so keep your guard up. Oh yeah, and make sure you use Magic Pot, preferably the non-auto version. It also helps to have your EX Gauge maxed and have some EX-boosting items. G_S is right, if you have any Resin items you can spare then use them; they won't break unless you win.
Sure people type paragraphs in the Spamzone, but when it comes to school or the more educated places, they just got three sentences. Ah the look of KHV on the moniter in the morning.