Hello guys!

I'm just wondering what champions could effectively counter kassadin, mordekaiser and bursty champs like le blanc or diana on mid lane since they are the only champions i have troubles laning up against.
Thanks in advance.

Try Gangplank mid against Kassa, works wonders (Be cautious in soloQ, sending AP top is not always a option and teammates might see you as a troll) - there's about nothing that Kassadin can do against him but hug his turret and hope to get some cs.

Cassi ****s Morde up, esp. if you get blue around 7~8 min.

Galio and Kassa > LB. Build bit tanky like 2-3 dorans + abyssal on Kass, or Chalice and stuff on Galio.

Morderkaiser requires champions that deal burst damage in order to be countered, for example Caasiopeia. Furthermore, the trick against mordekaiser is simply fighting outside the minions to avoid giving him that extra shield.
Le blanc is a nuke/assasin champion which clearly focuses on her combo ability to deal tons of damage and then wait for her cooldowns to do it again and again. I'd simply prefer champions that can either sustain her damage while return some damage output to her so we're talking nothing squishy here, or you could go for same stereo type champion which deals tons of damage in a combo as well, like veigar for example.
Regarding Diana, so far I've countered her pretty decently with mordekaiser, since she needs that close combact fighting in order to perfect her damage output and thats mordekaisers thing. But keep in mind that changes might arrive to diana at some point so she is not a stable champion just yet.

These are my humble opinions, they've worked for me so far, but Im pretty sure there are other ideas out there.

So it seems i have quite a few champions to buy now since i only have cassiopeia and morde from what you guys suggested. Thanks for the quick responses

Galio can use his great tankiness and big burst to stay in lane forever, however he does seldom get any kills until later teamfights.
Talon can counter almost any ap mid, however there are some exceptions.
If good, Akali can also be a good pick.

Mordekaiser is not countered by burst, and cassio dont have burst. Mordekaiser is countered by big sustained dps, and cassio has it(spammable ****). About lb and kassadin.
Change your mentality. You don't have to counter him. They counter themselves. You just don't have to feed them. You can outfarm them with many champs and sit back, and pwn them later. They can't farm minions effectively, and they are bad lategame, in teamfights. So don't let them get kills, farm minions safely, b when needed, and wait for oppurtunity.
Best blind pick cassiopeia, since she can counter some champs hard mid, while not really countered by any. If you face dangerous bursters she can just safe-farm minions with q and w.

1. I did not say you can't go to other lane after clearing minions with 2 spells.
2. If your team is feeding like kassadin goes top gets double kill goes bot double kill goes top double kill, solo baron then you can't do much indeed. Are you that noob?
3. Kassadin is pentakill sure. He can only punish squishes like that, and they usually stay behind. His burst wont do much against the darius tank, and if he goes for you, he indeed burst you down to 40%, but he will get catched and die.
4. i can understand you are a kassadin fan, but the way to beat him is not to feed him(say ss for example) and if he don't ged fed he is by far less useful in teamfights than 7-8 other aps. Sooo, tell me exactly when did you see anyone pick Kassadin in tournaments recently? And when did they ban him?

sometimes you don't need to counter pick some champions. LB and kassa are easy to ourfarm and they are easy to harras early lv.
Vs morde there are a lot of ad vruisers who win ad from the ap cass and malza.
And diana is counterpicked by swain

You don't need to counterpick these champions , Just learn your current champions better and you should do fine against these guys . Hard counter for both Morde mid and Kassa mid is Sion AP mid . If you learn him he is pretty oldschool but believe me he shuts them down hardcore . He also is annoying for Ahri and some other mids . Do not pick him against champions with sustained DPS or you are screwed (ryze,cassiopeia,swain)