Hi, here it's Kailikus. Firstly I would like to say that this is my first guide on Mobafire and I don't expect much acceptation.I'm a Silver III player and I've started a few months ago, but I also wanted to show my knowledge about Xerath and give a "Low Elo-Vision" of it on rankeds and normal games :D
Secondly, excuse me about my horrible english, I'll try my best to be understandable and explain on the best way those things I want to show you.
And finally, you can let me some advices to improve on my guides. Enjoy it :)

PROS:
-Great poke
-High skills damages
-Has a stun
-Lot of range
-IS BLUE!!!

CONS:
-No mobility
-Squishy
-Much counters on mid
-Hasn't got face :$

Flash: is your unique way to escape from a gank or repositionate on a teamfight. PLay always with it. It also provides you an offensive option to reach those enemies are out of your range.

Heal: I really like xerath with heal because he really doesn't need an ignite to kill someone and this gives you more survability and you can also save one of your allies.

Ignite: usually I don't take ignite, the reason is simple, you won't be close enough (in normal situations) to the enemy and will give you extra damage that you really don't need

With Xerath is relative easy to rank. You need to know some things of him to rank and then is easy win :)

1st- Try to stay behind minions, away from the other mid and warding by saving you of ganks.

2nd-Farm and poke, on early you can't kill someone with your combo, you have got no damage.

3rd-Stay out the fights and do the possible damage.

4th-You're a late hyper-carry. Don't let the enemy team catch you and ace him. GG WP easy win :D