The one thing I cannot stress enough while playing Xerath is that you HAVE TO use the range to your fullest advantage, and you HAVE TO poke with your q! Otherwise you will most likely get demolished! If you can poke enough and get enouph ap, you are still liable to ganks and you will fall quickly! But when given the chance, pop your w and see if you cna land a spell or two. Casting your w doesnt require any mana! One last thing, if the enemies get low life, use your ult! They come down quick and hard. And remember your w increases your range on all of your abilities, even your ult! So even when they're far away, you can still get them!

I don't have runes because I don't feel like spending IP on runes if I can buy champions with it. Also, get your itmes as you see fit, and use this as your basis. I'm sure the needs of the game will change, not accounting for a build. So be creative!