Mel vs Xerath

Stay back and poke with Radiant Volley (Q): its projectiles explode in an area up to 950 range, and each hit applies Overwhelm.

Land an auto after each spell: Searing Brilliance (passive) adds three projectiles to it per spell cast, and enough Overwhelm executes the target.

Save Rebuttal (W) for a dangerous projectile: the barrier sends it back to its caster, and you gain a shield and move speed.

Hit your target head-on with Solar Snare (E): only the core of the orb roots, the aura around it just slows and damages.

Cast Golden Eclipse (R) to finish from range: it hits every enemy marked by Overwhelm, wherever they are, harder with each stack.

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.