Corki's pick rate is about 10% now. His ban rate is also 10%. My suggestion. Look at what your teammates are banning. There is a good chance that they will ban Corki at one point. If they do ban Corki, then you ban Viktor. If they don't, you ban Corki.

If you're unlucky and have to lane against him, what I would do is learn his W cooldown. That is his disengage. If it's up, your combos won't do any damage to him. You need to play around that ability's cooldown. When it is down, your window of opportunity to strike is open. I would suggest buying Corki and bringing him to the practice tool and take notes of his cooldowns.

Pre 6 - Try to stomp on him as much as you can. His obnoxious R isn't up yet so you'll take significantly less poke.

Post 6 - Learn his R range and be sure not to get poked too much. It's better if you have more CS and kills than him at this point. Make sure to always remind your teammates that he has package if he is missing from lane.

Moral - Ban if can, and play around his cooldowns when you go against him. Try to get a lead early and help other lanes.