Realistically, there is very little chance an ADC survives a good assassin on their own. If you get caught out, make the best of it, but don't waste any summoners or anything, unless it gets close or you can flash over a wall or something.
Generally speaking, you really need to rely on your support/tank members of your team to hide behind. Try and position yourself in such a way that the assassin has to go through them to get to you. Don't stand in front of your front line, don't stand in front of the turrets when under siege, don't face check bushes, etc etc. Positioning is a big deal when it comes to surviving for adc's.
But, for itemization, there are a few things you can do to help.
- Lets you survive the burst giving you a fighting chance afterwards.
- the passive on Phantom Dancer is a huge damage reduction and helps alot with surviving extended trades.
- this is high damage, high life steal is great for situations where you need to trade and duel a lot.
For Runes, focus on Attack Speed, Lifesteal, damage and crit chance.
A lot of people put armor and MR in their runes, this use makes no sense. It prioritizes an early game defense, when ADC's are mid/late game champions anyways. So it makes more sense to give their late game a slight edge.
For Masteries, go 18/0/12, and in the Ferocity tree make sure you select Vampirism, and in the Resolve tree make sure you select Perseverance.