This build is mainly focused on building AP to absolutely kill the enemy team from afar. First off I would like to point out that this build is aminly for mid lane but can also be helpful if paired with a tank or melee DPS character and might possibly wokr with a support as they can help you if you are in a pinch. Enjoy your killstreaks. :)
As always i encourage you to watch the champion spotlight before buying or playing as any champion: Play Video
**UPDATED:
CHECK OUT BUILD 3 IT IS NEW AND I THINK IT IS THE BEST ONE YET, BUT KEEP SORCERERS SHOES!!!

Pros:
- VERY AP
- a sufficient amount of mana
- mid-late game dominance
- easy harassment with combo of mage chains and arcanopulse

Cons:
- very slow moving
- squishy in the early levels
- no natural "get away" ability

I tend to play Xerath as a ranged, aggressive, carry champion with this build you can be fairly aggressive and harass the other team loads.

During the low levels from 1-5 I generally harass enemy champions and focus on last hitting minions because this build costs so much money and there becomes less time later on in the game to actually farm very much.

During the rounds when your level 6-11 I usually call for ganks on my lane or go and gank other lanes giving us a free turret and making the game go by quicker. Hopefully giving my team a win.

During the levels of 12-18 I begin to go for long range kills and tell our team to rush mid around the 25 - 30 minute mark. Then you generally hang near the back and help my allies out of tight situations, get loads of kills, and farm minions with my farming technique below!

You may want to start with a Doran's Ring instead of the amplifying tome and the health potion or you might want to skip the first three items listed and buy the boots of speed and three health potions depending on how you wish to play. I recommend this build for mid players. You might also want more cooldown reduction in this instance buy the Ionian Boots of Lucidity but this removes some of your magic penetration so be warned. Also you have to remember that you must sell your Doran's ring later in the game but it is very helpful in the early stages when your mana regen and hp are low. Also, you may not want the Hextech Gunblade, so instead buy Morello's Evil Tome or the Lich Bane and do not buy the Hextech gunblade or revolver.

Instead of flash you might want to use ghost to still move about faster and escape death for Xerath has no natural "get away" ability. Although he does get a movement speed boost after using Locus of Power.

Instead of exhaust you may want to use ignite to get a good chunk of his hp lowered or just to scare him off but I still recommend exhaust.

The only other ability I would recommend instead of exhaust is clarity only if you run out of mana really fast which should not happen with this build unless you love to use the mage chains and arcanopulse combo 24/7.

In team fights try to stay near the back and kill the enemies from afar. use locus of power then mage chains, arcanopulse, arcane barrage, adn then another arcanopulse to probably get the kill or kills.
BEWARE! at teh back you are the most vunurable to ganks from behind so be sure to place a ward near all possible ganking routes from behind.

you may not even need to use Locus of power but it is recommended because it has no mana cost and when it is over you get a free speed boost.

Xerath is not strong at jungling but your team might want you to pull a neutral monster otr buff monster to do this stand right in front of the monster and use your arcanopulse then run while casting until it has about half health and your teammate can do the rest now if you want the golems pulled (I say golems because it should be the only moster you should go for, besides baron and the dragon only with a team, its buff provides you with cooldown reduction so it is very effective.) ask your tank or melee DPS to pull the golems and when the golems are about half way tell your teammate to leave and use an arcanopulse and mage chains plus casts in between running from side to side to secure the buff.

note: you should not go for the CDR buff until you are at least level 3 so that you have multiple abilities.

With this build you can easily survive even if your health is low. simply have your opponent chase you when you have really low health. then use mage chains and arcane barrage. he will still try to charge you so exhuast him and then repeat the combo when you are near your turrent only DO NOT EXIT LOCUS OF POWER. intead lead him with an arcanopulse and if he is stupid enough you will get the kill with possibly not a scratch on you.

when you reach the point in the build when you have over 100 AP then here is what you do. You activate Locus of power whilt the minions are still lined up and shoot it right down that line. then you runn wiht your movement speed boost and activate another acanopulse killing all the minions except the tank in which case you last hit to get all the gold.

- When playing as xerath you may need to run away a lot be sure you run with a teammate so that you do not get ganked.

- when low to half way on health recall where you stand, unless your team is running away, to draw attention to yourself, if the enemies attack you you can stun the nearest one and run away having yourself or your team pick up a kill or two. If they do not attack then you get away and you come back and own their faces. :)

- at the beginning of the game be sure to save your health potions until you are below half health that way the enemy player(s) might chase you down to your turret and you can secure a kill.

- I can play xerath as a carry character, you have to control your team and command them to work together, ask for ganks, place down wards and cooperate as a team and you will succeed.

I hope you enjoy this build please leave a comment telling me how I can improve it. a like rating would be much appreciated. thanks! ;)

you can also visit my youtube channel where I post lets play and gaming videos and willl soon be adding lol to my video series' my youtube name is DaSonorous. please subscribe :)