Like I said, I usually have little trouble with them. How many times did your lucian use his ability? Once, Twice? Without Serpahs, or possibly Shojin he has to auto too many times to trigger ludens when dealing with assassins. Since he is only tier 2 he has 1080 hp, which is I believe higher than a full item Kass's (Ghostblade, RFC, IE) dps if every attack was a crit but not by much, so he dies to two crits instead of one (this would take minimum of about 1.3 seconds, I think, just for one character, using the dps of a champ with what is almost assuredly most of their comp relevant items up until the very latest of moments where they might have two full item'd units).
The reason I don't have trouble with gold assassins (as opposed to silvers) is that I usually focus on getting golds of the lower cost units, as against assassins the hp bulk they bring adds a lot of survivability. A gold Garen and Darius guarding Draven adds 2160 (in essense this is only 2160 in total over a normal silver pair) hp per unit to soak up crits from their item carriers. Which from either Kass or Kha is at least 3-4 seconds of survival per unit, once again assuming they crit every hit.
If everyone has silvers then their dps is slightly higher depending on what items you use (if you have hp then it becomes slightly lower), but when you look at gold to silvers (the point the other guy brought up), their DPS is effectively doubled against you, and the defenses you specifically focused on (guardian, noble) are nullified as the only thing that helps is hp.
So in this case the fact that they got to their golds, and you didn't made a big difference despite what you said, and using ability based dps that is slower than simple auto attacking works against you with tier/burst/defense negation, and I am unsure how red buff actively helps with anything the void/assassin comp brings to the table, as I rarely see them healing unless they put like a BT on rengar (literally never seen anyone put a heal item on a void champ other than once a warmogs on Cho, but who knows).
Finally, I pretty much never build Red buff, Ludens, or for that matter Statik Shiv as I see them as early game items you build to secure a gold lead that kind of fall off late game (or in red buffs case, just a niche item with how bursty carries are atm, used to deal with shape shifters, and healing items/champs).
Its 1 AM here, so read through that at your leisure I guess, but the TL:DR would be that in this case your defense choices were countered, them having golds and you having silvers made a huge difference, and I personally disagree with your item choices against them but that is more subjective, as I haven't tested ludens much at all.
'd I had FOUR
. My Pantheon had 
Jinx had Sword Breaker/Guinsoos.
