Valkyrie is a great farming, initiation, and escape skill. Use it wisely due to its long cooldown.
Valkyrie can be used to jump over terrain and walls in the jungle and with Flash, Corki can jump over 2 walls and escape most ganks.
Since Corki is a squishy AD carry, you should rarely use Valkyrie offensively. Use it to keep up range between yourself and the enemy.
Maximizing the damage and armor reduction of Gatling GunGatling Gun will require Corki to be facing and attacking the target constantly for its four-second duration. It must be activated carefully to reach its full potential.
The armor reduction from Gatling GunGatling Gun will be removed two seconds after the last tick of damage the enemy has taken.
Always watch your Missile BarrageMissile Barrage quantity before entering a fight so you don't run out of it early. If possible, prepare a Big One. (Have at least 3 stacks towards one.)
Remember that Missile BarrageMissile Barrage is an AoE skill, and you don't have to hit the enemy champion directly, you may hit a nearby minion for the same damage. The Big One does more damage and has a larger AoE radius.
Because of the increased damage from his passive, getting a Doran's Blade at the beginning of the game allows you to deal a lot of damage. Starting with Boots of Speed and three Health Potions will give you more sustain and mobility, which is helpful when fighting strong poke and early gankers.
Trinity Force is a good item for Corki because you can get a Sheen proc frequently with Missile BarrageMissile Barrage and then apply it with auto attacks. This will strengthen your mid to late game damage output (as your passive applies with Sheen) and will give you some utility and survability. (in form of mana, the Phage slow, movement speed and health). The slow from it is also highly useful for maximizing the power of Gatling GunGatling Gun as the ability requires Corki to maintain facing enemy targets for its four-second duration, when constantly kiting usually tends to be a necessity for survival as a ranged-carry in teamfights.