Xerath is a VERY underestimated high-ranged AP carry.
It has a very high range, and does massive AoE damage with your ultimate, especially at rank 18, mostly going 1-hit ko, but your cooldown is still extremely high, so it is important that you get 6 Boots to reduce your cooldown on your ultimate.
Don't forget, that this is a hidden technique and you shall NOT tell this bug to Riot, so that people could use this guide in Challenger Tier to earn No.1.
This also includes a very complicated tactic in order to achieve this, therefore you may continue with the guide.

You must start off without buying any items at the start. You should keep your gold for late-game OP items, instead of wasting them on useless ****.
You leave your base ASAP, and immediately Teleport to middle lane, and Ghost and run into their base.
IF this gives away first blood, your team would likely to rage at you, all chattinig "GG THIS TROLL", but this isn't a problem. Make sure to say it in chat, because everything people say in the chat is a fact.
Run to the jungle( lizard elder aka red-buff), and you should keep auto-attacking the lizard elder till you die.
After that, your team will start QQ'ing like a little kid, so just say "**** you I'll afk then."
Though keep in mind, you should NOT afk.
You should stay in the fountain, and wait till you have enough gold to buy 6 Boots, where you immediately buy 6 boots, which grants you insane movement speed.
After this, you should run into their base, where the nexus would immediately blow up by itself, due to your immense awesomeness, making it an insta-gg.
Keep in mind, this is not a joke and a 100% viable guide.

Boots is a crucial start, and is the most common starting build for every champion, due to its mobility it gives. Though, 6 boots is extremely underestimated, and provides ultimate mobility for backdooring and allows you to quickly position yourself well in team-fights.
This is also 100% REQUIRED to earn 99% cooldown reduction and for the insta-gg tactic, therefore you may NOT alter the items in any way, even though many high elo people tell you that you should alter your items depending on the ally or enemy team composition.

Self explanatory. Everyone knows that the ultimate (in this case, Arcane Barrage )is the best ability for every champion, even the godly Phreak says so. Some people don't know a secret trick to put all your points into your ult, as you cannot press the + sign. Though, if you use ctrl+r, you are able to get your ult at level 1!

The runes are also self-explanatory.
You want to maximize your utility, thus the Greater Quintessence of Movement Speed.
The 2nd important thing is the damage output, therefore the Greater Mark of Magic Penetration, Greater Seal of Ability Power, Greater Glyph of Magic Penetration .
The runes may be altered to your liking, as this does not link with the insta-gg in any form, though it is highly recommended that you follow this runes, and that you should NOT use any tanky runes, as it would make you dying with red and turret slower, which would increase the risk of the enemy champion killing you in the process.
Other good choices for runes Great seal of avarice and Greater Quintessence of Gold would allow you to have higher amount of gold earning, therefore would allow you to earn 6 Boots faster, which would increase the speed of beating the game.

Here is a screenshot to prove my immense skills.Keep in mind, that my only losses is due to the fact that our team couldn't keep our nexus alive till I get my 6 Boots.
(My lolking, I have a gold elo). http://www.lolking.net/summoner/na/20937227#ranked-stats
Please note that I have the highest win loss ratio for Xerath in the world, and I should be the role-model of every Xerath player in the world.
Bow down to me and my guide...!!

Don't forget, you SHOULD buy S.E.X. (Scorched Earth Xerath), for MAXIMUM ownage. It would help you lose your virginity AND win you the game!!!!!!