Trade with Xerath when his W or E is on cooldown. When he doesn’t have either of these abilities, he is vulnerable to attack.
Investing in sustain and early protection will help to deal with Xerath’s poke in lane. A Refillable Potion wouldn’t go amiss in this matchup.
For Utility Supports, take Relic Shield instead of Spellthief’s as it’s unlikely you’ll be able to stack this item effectively.
Xerath will often go to ward objectives alone. Try to ambush him and catch him out of position. Then use the numbers advantage to take the objective or start a team fight.
Do not let Xerath poke you down before a team fight occurs. Xerath’s goal is to delay a team fight for as long as possible while he pokes with his Q and W. Don’t let him do this by engaging as soon as you can.
Xerath will try to stay in the backline- probably behind the ADC. In a team fight, try to flank and take him down at the start. This will make it easier for your team to win the team fight as he would not be able to poke you down.
Once Xerath is level 6 and unlocks his Ultimate R, he will have a lot of poke and extra damage. Do not over stay in lane post 6 as he might be able to snipe you.
Xerath is good in team fights as long as he is able to poke and harass with Q before a fight starts. He is not very good in team fights if he is unable to poke beforehand. So, make sure you engage as soon as you spot him.
As Xerath will be building AP items even though he’s in the Support role, he has similar power spikes to Mid lane Xerath.