Focussing him in a teamfight would be the best method. Sounds to me that he didn't have that much damage on him, even though he had penetration. Get everyone to buy their own penetration and burst him down. Having some armor also helps somewhat. Letting someone get fed is a situation which pretty much means you're screwed though, so there isn't too much you can do about it.

Warden's mail/Randuin. Reduce his attack speed on hit.
Frozen Heart. Armor + attack speed reduction.
Attack speed reduction hits Xin ABSURDLY hard.

IMO

Warmogs + Thornmail

Then again...dat passive. Probably best to just lessen his AS as much as possible. Hope he isn't stacking phantoms

xin is a physical damage champion like lee sin, pantheon or vayne. So the best defensive option against a Xin is armor. items like thornmail, randuins omen and GA you should buy these and a few hp items. xin's dmg is very strong with his q. no matter how much u try with the AS from his w he will land the full 3 attacks with his dash, but u can still defeat him.
hes very vulnerable for CC effects. so make use of all of these.