Corki is a viable champion in any elo just because his strong damage output and his late game scaling meaning he is dominate the whole game if he is farmed and it doesn't if he feed because if he is farmed he will be able to do enough damage with his passive.

People who he can beat in lane:
Annie - Dominating her early game will make you shine late game just because shes AP and your AD
Ezreal - All you got to do it dodge skillshots for this guy and dont let him poke you
Jarvan - May have some trouble but you could kite him top lane by bush camping
Malzahar - He is troublesome but your poking at lvl 6 he dominate if you aren't in his range
Nasus - He is hard but if you let him farm he will own you, best way to counter this is when he comes to siphon you Q and auto attack him
Shen - He is easy and sometimes go top and if so just own him
Singed - He is one tricky guy but when he runs to poison creeps you spam all your stuff on him and he should back up as in stuff i mean auto attack/Q/E/R
Teemo - This guy its just this guy here pokes harder then you but when you hit 6 he will envy you until you die
Vladimir - Its possible to beat this guy early on, you may think he is hard because of spellvamp and all but just autoattk him when he gets in ranged to simple

People you can't beat in lane or will have a lot of trouble:
Sion - This guy will give you trouble i mean AP sion will bushcamp you hard if he is smart so you might have to buy wards to ward the bush or its feeding for you
Mordekaiser - Its possible to kill him but his shield and just his farming beats yours and his harassing just so good because he has a strong laning presence all game just like you
Nidalee - Her heals its just her heals. Call for ganks because this lane will push once you get low hp and care for her flash Q,E combo because her Q is a finisher and E is a AoE siphon basically
Tristana - Her range is op, her poke with her W will ensure your death if not her poking with E and auto attacks will kill you most likely be aware of her damage output lategame
Kassadin - You may be asking why he is hard to beat but your a spammer with your E so he gets his charge simply and just can burst you harder then usual but lategame he's weak if you got some sort of MR
Caitlyn - Just such op range when you try to auto attack
Ashe - Hard because when you go for last hits a good ashe will hit you once with her auto attack because her range is bigger then yours
Swain - Hard because if you have issues dodging skillshots sometimes it will end up in your death
Rumble - He pokes to hard needs nerf enough said just switch lanes all i can say
Akali - Her burst once at 6 is op may need to get some MR or just get some HP
Leblanc - Will nuke but lategame no worries just play passive until then

Marks:
Armor penetration is good all round. People may say get magic pen but that is garbage and being a AD carry means to get armor pen to burst down tanks or people with a lot of armor

Seals:
I get HP Per level because its good and makes you not as squishy which could help you late game
Attack Speed may be good but its preference

Glyphs:
Flat MR is just good so AP casters cant nuke you hard early game
Mana regen per level is also fine

Quints:
Armor Penetration x3
Or Flat Hp x3

0-9-21
Getting strength of spirit works good because of Manamune and hp regeneration is pro thats enough

Utility is good because of the cooldown and what it benefits you in buffs/regeneration/cooldowns

I REPEAT DO NOT LISTEN TO THAT ITEM BUILD. LISTEN TO WHAT I TYPE BECAUSE IT VARIES IN MANY SITUATION.

Doran blade start if going bottom with support

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Boots 3 hp pots if going top or mid

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Stack dorans(having 2 or 3 if enough) if you are farming but having to back in lane more often
or you buy boots depending on there team

If you are farmed or fed.... meaning you are like 1-0 or like 1-1-0 you rush manamune if you extra 500 gold buy dorans or ward your lane so you dont feed or get ganked

Then if you are having trouble dying by casters or dps melee then get Banshee-Atmas or Banshee-GA

If not having trouble throughout the game you get this :
Trinity Force-Blood Thirster-Infinity Edge-Black-Cleaver-Atmas- This all varies depending on what you need like if you need lifesteal you get BT, If you want damage Infinity Edge-Atmas-Black Cleaver

MAX Phosperus bomb
Then Gatling Gun
Then Valkyre
Get your ultimate whenever you can

Ghost Flash on any carry.
Sometimes i come to conlcusion when i see a hard guy im going against i get teleport flash
Also Exhaust-Flash if you know you can easily beat them with your combo and exhaust

FARM FARM FARM KILL ALL CREEPS IN JUNGLE ETC JUST FARM
THINGS TO KEEP IN MIND
MELEE MINIONS-
2 HITS ON TOWER THEN YOU AUTO ATTK IT
MAGE MINIONS-
AUTO ATTACK IT ONCE
LET TOWER HIT IT
AUTO ATTACK IT AGAIN

Always call for a solo lane unless your laning with a support and just farm away. Don't try to feed your opponent or it will snowball and he/she will kill your whole team

Let your support babysit you
If you dont have a support then come in the teamfight when everyone blown all there cc and so into that fight where they dont even recognize you
and they will say in all chat HotshotGG[Nidalee]: That Corki Oh Man

The potential reason why this build is like it because corki is a hardcore ad spammer even though half of his skill are Ap but that wont stop you from getting more damage constantly

This is my guide to Corki
Dont downvote just because you do bad- everyone does bad sometimes just gotta get better and then you will know this build is dominate
I only made this for CLG so it can look more professional and look like a good community to check out guides

MY TYPOS I ES LAZY SO I ES NO TRY HARD K THX ;D