What to do against fed assassins?

Build hitpoints and all in them, they have to reset spells after burst, so if you have enough to survive 1 burst you can beat them. Exceptions are the hyper mobile ones that will escape 90% of the time. In those cases it just means build the HP then take the burst and run for your life.

Play a champion with sustain and CC on demand and use them to initiate for your team. An assassin is nothing when locked down.

win once you complete Zhonyas/GA.

Stay grouped, preferably with someone tougher than yo . Even assassins with cleaves or AOE can't melt multiple targets nearly as well as an individual who strayed from the group. I've saved quite a few squishies from assassins just because I was standing so close to them. This is also an effectice way to shutdown assassins who overestimate themselves.

With that said, you don't want to travel alone if there's an assassin who's ahead. If you have no armor at all and walk around somewhere with no vision, you need to assume that doing that will kill you. If you're a support who's squishy, then just slowly expand your team's vision without going to deep, and pay close mind to where the enemies that you can see are.

1. Don't feed them

2. 

3. Outscale and win

Do everything they said above. And if that doesn't work... Start praying.