In that case, the issue is mostly that crytal and wardens synergies suck in this set, and i asume lux is your other light, which means you have no light unit that really uses the AS well.
It also comes down to items, zed without items, assasins and ele doesn't do much, lux is decent on her own, but without the right items it's not enough to carry your damage, and other than that, you had no DD. And sadly, TFT is like the Rift, tankiness means NOTHING in the late, it's mostly about damage, and the the mage/infernals 3 stars had plenty of that.
As for why wardens and crystal suck:
- His setup is nearly all magic damage, so the armor of wardens means nothing at all
- Asides from the mage casts, all damage is spreat out over many instances of small damage, dianas abillity is split in 6 damage instances, zyras plants are many instances of lowish damage, the inferno dot...crystal just does shit vs 90% of set 2 cause it's POST mitigation and very few champs really deal enough damage in one hit to cross the 100 damage line.
Tarik for example has enough armor with the warden bonus that an even 250 ad would not trigger the crystal effect cause post mitigation, it's only ~100
Again, the sad truth of it is that your setup just lacks damage, especially if diana takes out your lux, and crystal+wardens are amongest the weakest synergies, and scarner without predator is mostly a waste of space.
Riots balanceing is bad, and you happened to opt into a non meta comp...