Hope that you have a really good team backing you up. Hope he goes for the AP carry instead of you. Get a Lulu ult, shen ult, and/or a shield from Lux/Orianna. A fed zed is really obnoxious to deal with as the adc :(
Yeah, if he dives straight for you, you just have to hope that that meant he got up wished by the rest of your team on the way that you can at least trade. That's one of those situations where your positioning is terribly important, but if it comes down to fight or flee. You're not getting away. Might as well stay and output as much damage as you possibly can.
Also it depends on the degree of how big the Zed is, but a well times Barrier can negate a lot of his Ulti damage. Enough that if it's in the late game and you have a lot of life steal, you may be able to sustain through it and pray that your team peels well. But it's not guaranteed that you'll live, simply because he is an assassin.
Also, if he's super focusing you, tell your support to specifically save their exhaust for him during his main combo. That again can really high ten your chances of surviving initial burst and living long enough to punish him for diving.