Karma vs Xerath

Hit the enemy with your spells: Gathering Fire (passive) lowers the cooldown of Mantra (R) with each damaging ability.

To poke, cast Mantra (R) then Inner Flame (Q) at a group: the explosion leaves a ring of fire that slows and deals more damage.

At low health, cast Mantra (R) then Focused Resolve (W): you restore part of your missing health at the start and end of the tether, and the root lasts longer.

In teamfights, cast Mantra (R) then Inspire (E) on an ally surrounded by the others: everyone gets a shield and move speed, to engage or to escape.

Keep minions between you and Xerath: Shocking Orb (E) stuns the first enemy hit, for longer the farther it has traveled.

Don't stand still, and change direction often: Arcanopulse (Q) hits a target moving forward or backward in a straight line much more easily.

If you can engage, go in while he charges Arcanopulse (Q): he slows down by up to 50%, and he has no dash to escape.

If you have CC and can reach him during Rite of the Arcane (R), use it: he channels the ult, and interrupting it stops his shots.