which champ take to beat kassadin on mid?

Don't be AP.

If you're any of the ranged AD carries you can just poke him over and over and hopefully you've won the lane before he hits L6 and can harass back.

Think kassadin struggles agains heimer. not sure tho

Morgana is good Kassadin counter

Miss fortune

Well, Caitlyn is really good against Kassa, since the range. <-- Personal opinion
As other/s said before, just don't be ap, (mostly) any ad carry will do the job <-- General truth

I'd say Heimer(kinda), Morgana and Yorick.

Pick kennen and say GG kassa

Galio!

I've never tried this against a kassadin but it might work:

Pick orianna, her long range should be good to harrass since kassadin is a melee champion. If you get him to "waste" all of his Q's to last hit minions then you should be in a good shape. If he tries to melee attack them just poke him immediately. As said, never tried but I can imagine this working well. Also remember that if kassadin uses his combo he has cooldowns, so try to poke him right after his combo.