Build and Runes
Avoid extended fights because his Gatling gun will tear your armor.
His Valkyrie can be interrupted mid-flight by knock-ups (e.g. Alistar’s Pulverize) and displacements (e.g. Alistar’s Headbutt).
Stop him from backing in lane will immensely help you because Corki’s abilities cost a LOT of mana.
Valkyrie has a large cooldown at earlier levels, making him vulnerable if he’s used it recently.
Every third rocket Corki fires deals more damage. Keep track of the number of rockets he’s fired to avoid getting hit by the rocket or splash damage.