Adc's shouldn't be trying to 1v1 assassins in the first place. Adc's provide sustained damage, assassins delete squishies.
An adc like Ashe who relies on mostly autoattacks will always have a difficult time dealing with Yi. If he uses his Alpha Strike as a gap closer, does not have full health, you might be able to stun him with your ult and defeat him. You'd have an easier time with Graves, but...
It's just not favorable in a 1v1. That's why you get a support, and that's why tanks are useful against assassins in such a manner.