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.

Xerath's passive is Ascended form.
"Xerath receives bonus armor equal to 15% of his ability power."
Xerath receives an increase in armor scaling with Ability Power, which would be awesome for teamfights, but not for the insta-gg tactic.
At the start, you would like to die to red buff and turret as fast as possible, thus, the magic penetration runes instead of ability power, as it would increase your armor, and would increase the risk of dying to an enemy whilst tower diving/dying to red buff.
Xerath's 1st ability (Q) is called Arcanopulse
"ACTIVE: Fires a long-range beam of energy in a line after a 0.75 second delay, dealing magic damage to all targets hit."
For a normal Xerath, Arcanopulse is maxed first, and is your main ability, though, for this insta-gg, you would not require this as you would be maxing your . Arcane Barrage (Ultimate) to rank 18 (or 17 or 16, which I would explain in a minute).
Your 2nd ability (W) is called locus of power.
"ACTIVE: Xerath anchors himself to his current location with a 0.5 second casting time. For up to 8 seconds afterwards, he gains increased range of all his spells and bonus magic penetration, but cannot move. Xerath can deactivate the ability at any time after 0.5 seconds of casting it, and gains 35% movement speed for 2 seconds when doing so."
This basically temporarily prevent you from moving, (allows you to autoattack), and while casted, increases your magic penetration and ability range.
While in the cheat-sheet, I have put 18 points in Arcane Barrage (Ultimate), you may take 1 point in Arcane Barrage, just for the range and magic penetration, as Arcane Barrage does 1700 Magic Damage in one hit (5100 Magic Damage when all hits excluding ability power), therefore, you may want to take one or more points in locus of power in order to increase the range and magic penetration.
Mage chains is your 3rd ability (E).

"ACTIVE: Deals magic damage to an enemy and marks them with Unstable Magic for 3 seconds. Unstable Magic will trigger if a marked enemy is hit by one of Xerath's spells, stunning them for 1.5 seconds."
You may also take one point in this ability, not for the damage, but for the stun it gives.
When combo'd with the huge ultimate ( Arcane Barrage), it allows you to easily stun one enemy, and would be great crowd control for the team.

Your final ability, your ultimate is Arcane Barrage

"ACTIVE: Calls down a blast of arcane energy, dealing magic damage to all enemies in a target area after a 0.5 second delay. Arcane Barrage may be cast another two times, with no cost, within the next 12 seconds before going on cooldown."
This is your main ability in the insta-gg tactic, and does massive aoe damage, great for the Teamfight. If you could land all of these on all the enemy at rank 16, it is likely to be a guaranteed pentakill due to it's massive damage.

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!
You may also take locus of power and mage chains for the increased range, magic penetration, and the crowd control it gives you, which should also be self explanatory.

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!!!!!!
NOT TO MENTION, BY USING S.E.X. YOU GAIN INFINITE AP!!!!

http://leagueoflegends.wikia.com/wiki/Xerath was used for ability info.

I hereby declare that I do not take responsibility for any losses from this guide, and show no guarantee in the accuracy of this guide.