This "Auto Death" build is best for those who don't like to rely on activated items such as Sword of the Divine. I find that relying on activated items is wasteful and can result in activations that you don't get the full benefit from. Activated items can also cause apprehension of initiation if they happen to be on cool down. With this build there are no activations and you get the full benefit of your items at all times.

Summoner Spells

Since Xin lacks escape mechanisms I prefer to use the Summoner spells Exhaust and Ghost. Both can be used defensively as well as offensively, which serves to augment both your chasing ability as well as providing some escape possibilities should you run into trouble.

Runes

Greater Mark of Desolation

Armor penetration marks are, to me, the best choice for this slot. Attack speed marks provide 15% attack speed, but we're already stacking a ton of attack speed items in this build and your total runes and masteries should provide 31 armor penetration, enough to secure you some early kills and help out with tanks later in the game. Armor penetration also scales much better than attack speed in late game.


Greater Seal of Alacrity

Attack speed seals provide a somewhat minimal boost but there's nothing else really noteworthy to use in these slots. If you feel you'd rather be tankier, go for some Seals of Vitality (health per level) or armor per level.


Greater Glyph of Celerity

I prefer cool down per level glyphs because Xin is primarily a mid to late game champion. The benefits of flat cool down glyphs are equal to cool down per level glyphs at 13, which takes about ~20 minute to achieve. Most of my games last 35-50 minutes, so it only makes sense to use cool down per level over flat cool down.


Greater Quintessence of Desolation

More armor penetration. I find that the amount of armor pen from the marks and quintessences along with The Black Cleaver provide the perfect amount of penetration.

Masteries

Standard 21/9/0 mastery build. Provides a little bit of a defensive boost while augmenting your damage output. Going 21/0/9 for a boost to your XP gain is also a reliable mastery set up, but I hate the fact that 4 mastery points become useless after you hit 18.

Berzerker's Greaves

I usually pick this when my opposing team doesn't lean heavily toward AP or AD. Obviously, if 4 out of 5 of their team is AP or AD, I will choose Mercury Treads or Ninja Tabi, respectively.

Black Cleaver

Perfect combination of damage and speed. Your already fast attack speed will help add the maximum stacks quickly.

Phantom Dancer

I'll usually get this if I'm doing well already. If I feel like I'm dying too much or my opponents are prone to focusing me, I'll get a Frozen Mallet before Phantom Dancer.

Frozen Mallet

Frozen Mallet works well in most situations as your 4th item. However, there may be some times when you want to rush a Trinity force first and forgo a Frozen Mallet all together.

Situational items

Fifth Item
or
(Bloodthirster / Madred's Bloodrazor) - You now have terrific damage, attack speed, health, and now life leech. With your passive and this item you can out heal many other melee characters' damage 1v1. If my opponents have more than 1 or 2 tanks, I will buy a Madred's Bloodrazor, otherwise I get Bloodthirster.

Sixth Item
or
(Banshee's Veil / Trinity Force) - If my opponents have a lot of CC, I will usually pick up a Banshee's Veil as my 6th item. If not, I will get a Trinity Force. Trinity takes your burst damage to an entirely new level. You should be nigh invincible at this point. A lot of people use Infinity Edge as their 6th item but with Xin's quick cool downs Trinity does FAR more damage.

Early Game

Start out with a dagger. Next build your boots. From there make a zeal and then a phage. Notice that I don't start out with a Doran's Blade because it can't be used to be made into anything else and becomes a waste of gold during mid-game. I tend to play early game a little defensively and try to get a gold / XP advantage on my opponents by clearing the minion waves to the middle of the lane and then falling back to the jungle (if there's no jungler) to pick up some extra gold and XP while my lane mate gets extra gold / XP for clearing the lane solo. Xin isn't a particularly strong early game champion because of his dependence on items, so you should definitely play defensively. Your early game expenses come to 3430 gold.

Mid-Game

Mid-game begins when you make your first large purchase. This could POTENTIALLY be upgrading your phage to a Frozen Mallet or Zeal to a Phantom Dancer, based on the situation, but for the most part you should go for the B.F. Sword and later upgrade that into The Black Cleaver. The reason for this is between your boots, zeal and phage, you already have decent attack speed, critical chance and health, and are only missing a large damage boost, which B.F. Sword provides.

Your next mid-game purchase will usually be Phantom Dancer, since it will add even more speed and augment The Black Cleaver that you made previously. Occasionally it can be beneficial to upgrade your Phage into a Frozen Mallet prior to upgrading Zeal to Phantom dancer, particularly if the tank on your team isn't very good. The additional health, I've noticed, acts as a psychological deterrent to prevent your enemies from attacking you in lieu of a squishier target. If you're performing well already you should get the Phantom Dancer first and pick up the Frozen Mallet afterward. Your total gold spent should now be 10,130.

Late Game

Late game begins when you've finished your core build of Berzerker's Greaves, The Black Cleaver, Phantom Dancer and Frozen Mallet. Now you need to take inventory of your opponents and allies. If the opposing team has 2 or more high health characters (not necessarily just tanks, e.g. Vladimir) or one fed/skilled tank, buy a Madred's Bloodrazor to hit them for 4% of their health each attack. If your opponents don't fit this criterion, buy a Bloodthirster. This will help you tear squishies apart and mitigate their damage output by augmenting your life steal passive.

For the final item I usually go with a Trinity Force. It's damage output is incredible and helps you really rack up the kills during a 45+ minute game. If the game is a close one, I usually get a Banshee's Veil to help ensure survival (by mitigating CC) and not spend over a minute dead while the enemy team goes unchecked. Your total build cost is 17,400.

Once you get good at Xin and begin to dominate games you'll notice that your intelligent opponents will attempt to counter you. There are several counters to Xin, but all of them are counter-counterable as long as you play intelligently too.