“Really Squishy, he needs to get up close if he wants to do serious damage, which favours you hard. A good Karthus will farm with Q and wait for late game however, try to prevent that.”
“Karthus comes as an extremely easy match up for Leblanc since he struggles to hit many abilities on her at all. Leblanc just has to hit full combo's and dodge all of Karthus' Q's and she straight up wins.”
“I out-range him, so I just poke with E without going for passive procs and putting myself in a position to lose a trade. I'll build Zhonya's for his R, but there's no point rushing it as it doesn't deal that much damage early on. If he's some sort of god that lands every single Q, I'll get some Mercury's Treads. He's fairly immobile and pushes easily, so my jungler usually has no issue ganking him and slowing him down from scaling into a late game monster. I'll also spam ping when he's close to 6/gets 6, so my teammates are reminded to back or avoid fights if they're low on HP.”
“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).
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).
Be careful of isolated Q (just like Kha'Zix). Play around minions and try to get a full combo on him. Remember if you kill him to back away as soon as possible because of his passive. Always keep in mind that he can ult when you are low hp or your team, so use pings to communicate. Ping when he makes level 6 so your team knows that an ult might come. Roam but keep in mind that if he has ult he will use it.
Try to go for the kill as often as possbile. Play with Ignite and get a stopwatch if you want to deny at least an ult.”
“Mainly a poke match up/dodge match up. Your level 6 beats his though, play aggressive into him with Q-E to combo with your W then R at level 6. Buy zhonya's if you think you're taking too much damage from his ult.”
“I don't think anyone really plays this champ mid any more but it's a free win if you go against him for obvious reasons. Just play very aggressive but smart and you'll win.”
“Personally on the surface this matchup is one of my mental blocks. But Karthus himself is quite predictable. Sit near minions so you take minimal damage. He will outpush you but run out of mana fast. This is almost like a dumbed down level 6 Anivia, so with this being said, his low mobility allows for amazing setup ganks. And he auto pushes into you. You can try to look for ganks but it will be quite hard with his pushing power.”
“For Karthus, it's really important to not get harassed too much in early. A lot of Karthus players are trying to go on your face and spam their Q on you in order to drop you down. If it happens, it's quite good to get some jungle ganks if possible, otherwise this lane might turn into nightmare. On the other hand, once when you hit level 6, you can mostly use your combo on him and kite him in your E because once again, he has to walk up to you if he wants to do something. In that moment, you are the one who dictates the rules of trading. For lategame it's quite good to have Banshee against him or Zhonya to avoid his ults, otherwise you might have a lot of problems to survive in teamfights, since your ult can chunk you down really badly with high amounts of AP.”
“Buy boots first item to dodge his q. Look at the boots section to see which ones you need.”
“Karthus' Q is super easy to dodge with boots, and he is super immobile and squishy. Beware of his R though, since it can easily kill you if he has Dark Harvest. If you are having troubles with him, an early EoN is great.”