how do i counter mordekaiser + soraka bot lane duo ?

That's probably not a duo you're going to beat by straight up fighting them in lane.

I'd suggest shoving them under turret and making them miss CS, then either try to get ganks and take out Soraka to leave Morde vulnerable for a time, or simply waiting until it's time to group up and try to out-teamfight them.

'Course, you could always try to play the split-push game, since any team with Morde on it is going to have trouble with objectives if he doesn't have Dragon ghost.

you just need a consistent amount of damage, honestly, I'd say your best best bet is taking a cassiopeia support with a twitch.