So Kassadin is a pretty good pick into Yasuo....

Take Q level 1, W level 2, E level 3 then max. Use your Q on cooldown to keep his passive down. Level 1 is a bit rough if they take E first, but once you get your Q and W you can keep his passive down, and AA-W should be more burst then he can put down plus empowered autos trade really well. Once you get your E it's a piece of cake. Q to keep his passive down, AA-W if he tries to go in, then E and walk away. It's worth noting that E goes straight through his shield, and if Yasuo isn't casting spells he isn't bullying you or playing aggressive. He's all about the spell spam so it's pretty much always up.

Naturally post 6 if he lands a knock up shit gets real dangerous but unless you are shoved way far up it shouldn't be the death of you. Once you come down R away then E as he tries to follow. Jungle ganks are dangerous pre-6 as always but he should almost always be pushing into you. Bonus points for low ELO Yasuo's e'-ing too close to tower. I've gotten a lot of first bloods like that. I generally take ignite in lane since it's kind of necessary to secure early kills, but exhaust also works great if you really want to double down on sidelining Yasuo. Just be weary that he may still have an advantage with ignite.

I usually go 12-18 (thunderlords) for masteries, but 18-12 (Deathfire Touch) is also an option since you will be harassing a lot. I like the quick burst of Q AA-W or E-AA-W to get a proc since bursting and disengaging are your best bet in fights. Yasuo can and will win a straight up AA range duel, especially once he gets his crit item, so do NOT attempt to duel him. Go in burst, get out repeat. If he falls even a little bit behind it becomes very hard for him to respond to that.

Now laning is one thing. What about the teamfight phase? Yasuo is a late game carry after all, and that ult is pretty game changing. Naturally its hard to talk about teamfighting in a 1v1 vacuum. Team comps, skill, other lane's performance all factor in. What Kassadin offers is a mobile flanky assassin that excels at getting to the back line with his ridiculous mobility. He does not offer the AoE oh-shit moments Yasuo does. But with adequate vision control and items he can very often knee cap the enemy team before they even get a chance to set up for a Yasuo team fight. Vision control is key, you need the enemy map to be dark and yours to be light. Farsight Oracle is a pretty damn silly right now, and great for picking out targets of opportunity. Otherwise you can take sweeper to help keep the enemy in the dark. Double wards can work but the sheer safety offered by FSO is hard to pass up. Your target should be the enemy carry, but any squishy is a boon, supports can make great targets especially squishy utility types. Addendum, if you see a Soraka KILL IT DEAD, even if you can't quite kill her she can't heal as effectively when chunked down. Trust me you don't want to have to deal with her topping Yasuo or the carry up.

The gist here is to never give the enemy a fair straight up 5v5 brawl, even a shutdown Yasuo can find an opening in such chaos to get back into the game. Catch enemies out, knee cap the carry, win the game. You make a decent split-push threat when you get your ult cooldown low enough so they can be a good option to force the enemy team to split up.

Let’s talk build. RoA first ALWAYS. You need to get this finish ASAP to start it stacking. It may be tempting to deviate and build some other item components like Seekers, but you must resist the urge. A seekers is a very short reprieve against Yasuo, and without the AP, HP, and Mana RoA gives you won't be effective at killing anyone. Your mana costs are huge, you need that big pool ASAP to begin functioning. That said, an early boots can work wonders chasing down a injured Yasuo or buying you time to kite away. Fucker is fast. After RoA it's your choice. Archangels gives a LOT of power but its another big time commitment and its very possible for the game to be over before it really comes online. I personally don't buy it, but the argument for ultra-late game scaling can be made. My second item varies between Lich Bane and Zhoynas. Zhonyas got shit on in the pre-season patch but works so well with Kassadin's play style. If Yasuo is still a big threat then its a worthy second bye, just remember you're missing out quite a bit of damage. Lich Bane is Kassadin's bread and butter. It gives his W a 1.3 ap ratio, it syncs really well with his play style, and everybody loves CDR. Second or third item always. You can start blowing Yasuo up with this thing. RoA-Zhonyas-LichBane are your core. Deathcap and Void Staff are extremely good options to keep blowing people up, but it's personal preference.

Alright guys, it’s been fun, please feel free to tell me where I'm wrong and how bad of a scrub I actually am. If you find this useful feel free to tell me that too. Also Rito please rework Annie that fucking bitch is unfun as fuck. She either blows you up in a stun or doesn't, there's no outplay on either side. She's not OP just really fucking obnoxious. Especially for Kassadin :(
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