How about we do some math, hm?
Let's say that we are being ulted by a level 18 Zed, with 400 AD (including W passive). This Zed ults, lands his Q, E, and a basic attack, not including his innate passive.
His Q deals 522 damage, his E deals 410 damage, and his basic attack 400 more, which totals to 1332 damage.
Zed's ult deals 100% AD plus 50% of the damage dealt to the target, so 400 + 666 (oh wow, not intentional, but OK) which is 1066.
So no, if you get ulted by a level 18 Zed who lands his full combo, you fucked, 50% is too little for this Fed Zed.
Of course, this is completely exaggerated to prove that you should probably be building tons of fucking armor to deal with this insane damage from Fed Zed over there.
QSS won't save you from the other 1332 damage this guy is throwing at us.