What am i supposed to do? to not get oneshotted as a tank

i was playing ornn and had around 500 armor i had conditioning and overgrowth around 3500 health nearing 4000 + ninja tabi

enemy team has a lucian with ldr , bc , bork. and for runes he has pta and coup de grace.

how does he just walk up melt me away like butter? he kills me in less than 5 seconds. How am i supposed to build against this? okay fine if it was vayne i wouldnt really complain because her kit is broken and if she was in the same spot probably kills me in around 2-3 seconds. but what am i supposed to do , whats the point of building defensive when im not even useful enough to even act as a distraction or a damage absorber. [sg-janna]

I checked the self mitigation damage after the game , correct me if im wrong isnt self mitigation damage the the damage you would have gotten say u didnt have armor/mr death dance etc ? an ornn with 3 full armor items + tabi mitigated 54k damage (now keep in mind that ornn has a passive that gives me 10% more armor and mr because of my passive ) the damage taken myself was 32k. I reduced 54k damage? but i still get melted?. Also for a bonus , tristana took 95% of her hp from a lucian who autoed her once (plus his passive shot so in total 2hits ) thats fun. Why am i not rewarded for playing defensive ? i cant fight as a tank because he just outdamages me and any dmg i do is basically negative [sg-ahri-1] Help me.... what do i do , i cant kill i cant stay alive as a full armored tank against a full ad team (they only had nami so like 10% of the dmg is magic?)[sg-soraka]