W is easy to hit and give to enemy slow. So you can combinate it with Q.

Xerath is poking champion so when you are trying to kill someone with less hp than he you must hitting him and at the same time you must slowly go under turret and then when he has 35% hp, snipe him with ulti. You must be only under turret and he cant do anything. When he try something, use stopwatch or hourglass...

Poking with Q always try throught minions to farm.

Stay out of combat and poke... Everytime has xerath one of best damage in teamfight. Only throught you skills. You must stay alive and Q'ing.