Xerath i have found to be a powerhouse at AP, with his ult having a total ap ratio of 1.8, and his passive makes it hard for AD to hurt him while he builds glass cannon, with his long range spells makes it easy to take out their carries, MR is your bane like any AP carry, but unlike other AP carries where AD chew you up, Xerath with this build has over 230 armor = about 72% ad damage reduction, with about 3k of HP, making you a "anti AD tanky glass cannon AP mage" and you will be nuking squishys with 2-3 "r's" and ur E Q combo

xerath shines most in mid because its just a 1 on 1 which means you save mana by focusing on 1 target if you keep hitting. xerath in early its best to poke when you can when you are sure that you can hit them, missing is bad = wasted mana if you miss minions too.

when you get your e, start using the e q combo, and run back, the stun makes sure that you cant be counter poked. when you get your R, and they are about 1/2 down, use E R Q R R if you are close, if they are barely within your w's E range, then w e r q r r and you maybe able to use another q with this combo = more likely they wont escape your massive range

this is when xerath starts to shine, try to get mid down, so you can start ganking, with your 1.8 ap ratio ult, you are very scary, and if they dont run its either

1 they are stupid and will die

2 you or your team is about to die and they want a kill before they die

3 you are getting counter ganked

after getting top and bot, farm as much as you can

this is when people start to catch up and xerath starts to lose a little shine if they start to build MR, but when you get ur second death cap(sell the tome), you ap should be with baron and magic exlir >900 with over 230 armor with "meh" MR, MAGES kill xerath more then anthing in late game, so after you take out the carries, take out the annoying mages with your mighty LIGHTNING BOLTS AND THUNDERBOLTS, and watch them get fried to a crisp before they die.