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.