He had all 3 voids as assasins(i asume) and 3star(so he's just as lucky as you if not more with his setup), which is a massive hardcounter to your setup.
Your setup has tons of defense, but no damage at all. And the thing with 6 assasins is that they deal so much damage, especially as 3 star, that they can actually punch through defense. Unlike the normal game, assasins here tend to counter tanks instead of getting countered(if it's just tanks) cause they have the by far highest dps with their crit.
But even more importantly, half of his setup was VOID, so truedamage which means they just don't care about the nobles and knight buff anyway, so what you effectively have left is a random bunch of 2stars that die to quickly to get use form the on-attack heal.
So even if he only had 1 yomuu's and thus just the 3 assasin bonus, the truedamage is enough on 3 lvl 3s to deal with your setup, cause you simply don't have damage to kill them(unless you have somne really absurd items, and even then, vany ewould be the only actual carry in that setup and if she gets assasinated it's over)
Void assasins are currently prettymuch on top of the foodchain atm, especilly if you can get more than 1 ghostblade. And getting all 3 of them 3stared is just gg...