It is a solo experience. If the player is good and can do the combo, opponent's actions don't matter. But if the player cannot do the combo, then he is useless in the game. So there is not really any counter play to speak of.
I can counter assassins easily, even if they pull off their combos easy.
I play Ziggs top/mid and just poke 'em down.
Whenever there is a Yasuo mid (even though he seems to be more of a duelist like Fiora), I play Karma.
When there's a Camille top (seems to be more of a duelist honestly)... I play Illaoi.
Or you can learn to counterbuild and actually ward and have any sort of sense of caution and map awareness instead of just whining there's no counterplay.
[deleted]
umm yes there is? dodge the 
Q/
spear/ 
charm and u win? skillshots = counterplay.
granted if they dont have skillshots... yeah you have a good point... but not ALL assassins have "zero counter-play".
