Once you have Cap, you have 400 AP easily with runes and masteries + RoA & Codex. Xin'll have his items but he generally itemizes ASpd for obvious reasons, which was accounted for already. He may and should itemize defense in such a scenario but at that point the math gets too complex to truly be worth it. All I can tell you is that in my practical experience, you should have no trouble laying a hurt on him as long as he isn't able to stack only MR (that is, your team contains multiple physical damage dealers too) and after your first volley, you can easily land a second one before his cooldowns are back up.
Xin can't really escape; again, he's slowed and he's forced to commit to do ****. Xin's biggest weakness is that he's so all-in; once a Xin engages, anyone with a slow can prevent him from disengaging and anyone with a strong slow can prevent him from continuing to engage either putting him in an awkward position against anyone that doesn't instantly die or want to fight him 1v1. Xin wrecks in the fair fight but we're specifically talking about not fighting the fair fight here. Fighting fair against champions who can only engage in melee is stupid.
The map isn't only about melee champions. Melee champions are terribly suited for capturing defended points and quite poor for defending points too. Melees' niché is fighting over uncontrolled points and in the jungle, and during dives; they have their place but a team of only melees is, put frankly, quite terrible against a rounded team.
He will get his E back up in 5 seconds
Don't believe me we can 1 v 1 on dominion right after im done reformatting, ill play X to the Z and you can play Janna we will fight in the center