Nami vs Xerath

Cast Aqua Prison (Q) on a slowed or immobile target, or where they're heading: the bubble lands in an area and stuns every enemy on impact.

Cast Tidecaller's Blessing (E) on your ADC just before they trade: their next autos and abilities slow and deal bonus magic damage.

Cast Ebb and Flow (W) so it bounces from one champion to another: it heals each ally, damages each enemy, and only hits the same champion once.

Cast Tidal Wave (R) to engage or to break an enemy engage: enemies hit take a knock-up and a slow, which lasts longer on those farther away.

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.