Cassiopea's miasma completely disables your E from being cast and it's a large area which doubles the damage of her poison, personally I think hullbreaker is a necessity and you can look for all-ins with the flash combo but it's very risky it'll workout, if you start losing in this lane it's likely you'll be out for the rest of the game, ask for some jungle assistance but don't be decieved if you get ahead a 0/6 Cassiopeia with Liandries will 1v1 you regardless of how ahead you are, HB/Mercs will give you fair standing against her but again, this is your second worst matchup, I would encourage dodging. Urpog says “Toplane: Cassiopeia's your second hardest matchup behind Anivia, the same can be said for midlane. She will follow up with miasma so you can barely move and then kill you.

If you get stunned by your ultimate, there's a super high chance that you're dead. If you succeed in doing that, then you're going to win all the trades. It will only slow you instead of stunning.

She will most likely use her ultimate when you're going for the knock up with Q, so MAKE SURE to turn around. With the second engage you can go for the kill. Cassiopeia maxes that spell out last, so your E will end up being on a much lower cooldown. Her miasma has a 24 second cooldown early game. Go in with E + Q3 and then hit her as much as possible before she's able to use abilities again, and BACK OUT with E2 before she uses her miasma. If her miasma is up, you want to go for super short trades where it won't screw you over. Rush tier 2 boots asap so you can dodge her skill shots more easily. Her ultimate will stun you on the spot if you're facing in her direction, and she will shred you to pieces with twin fangs if you're poisoned. Yeager says “Cassio can screw you over with her miasma, making you unable to use any movement abilities/summs including Q3, E1, E2, R and flash. If you hit your W E Q's combo you have a good chance at winning but otherwise I'd just push and roam because it's a rather annoying and difficult matchup (kind of like Azir).” A good player might also try to kill you pre 6 and kick you out of the lane. The impossible thing to dodge is the R->Flash ult - so keep in mind that a good player might do that to kill you. (You need some practice for this but in time it becomes easy). Her ult is highly predictable and you can dodge it if you turn around right when you see the animation starting. You can play the range advantage with your Q but if you get close be careful since her movement speed (phase rush maybe) can make you miss your W. Early try to get boots as fast as you can to dodge as many Q's as you can (since Q enables her).

Rather difficult matchup if it's a very good Cassio. Start Corrupting if you think you will not hit your W's (or you will get poked down) and you rely more on wave clear/spammy poke (you will be way more squisher but more durable - this is recommended if you are sure you will not get hit by CC spells and instantly die). Drewmatth Taliyah says “Start Doran + 2 Pots if 1) You think you can outplay her and hit your W's to generate a kill at levels 3-5 or 2) You think she will burst you down post 6 (get an extra ring).

Cassiopeia adc