What are your options against AD assassins as an ADC?

So I'm still in placements so I don't know who it is sticking me with but I find that as an ADC I get no peel at the moment and ad assassins run a train on my face starting in the mid game (most specifically Zed/Rengar). What would be my strategic and itemization options in this situation? With AP I can at least buy a Maw and remain fairly offensive in my build. My problem is with AD assassins is they generally walk/stealth/blink right past my front line who are diving in at the enemy caries and murder me regardless of how far back I am. In fact if I am too far back it's even worse because then the team just completely ignores the assassin. and they get an even more free kill.

I generally run Ashe when I need to ADC by the way. I just want to improve my play to hopefully make up for any shortcomings I may have from my team.

Thanks.