Lethality and You (TL;DR at bottom)

Lethality isn't broken.

The 3 items that give lethality give 18 each. 18 * 3 = 54 Precision gives 6 lethality. 54 + 6 = 60 Runes give 24 lethality. 60 + 24 = 84 lethality (flat armor penetration)

has 84 armor. Without an investment into armor, she effectively has 0 armor if attacked by a lethality built champion

Here's a list of champions with less than 84 base armor at level 18 in descending order of armor value down to 65.5 and Mini-Gnar

If your champion is on this list, you effectively have 0 armor against lethality builds if you don't invest gold into the armor stat. This means you have 0% damage reduction, translating to you taking full damage from every physical attack.

I'll use as my example to give you an understanding. Wukong's base AD at level 18 is 114 65 55 55 40 80 (sometimes I go instead) That makes his total attack damage = 354 Transalation = You take 354 damage per AA

Q = 539 E = 531 Duskblade = 360

EQ with duskblade proc'd = 1430 damage. You're going to take the full 1430 damage The actual proper combo is E > AA > Q > AA

E > AA > Q > AA > = 2138 damage

Without an investment into armor/health (which most assassins/mages/marksmen builds lack), there are only 11 champs with the base health to survive that;

This doesn't include the following; 410 true damage from 250 magic damage from Decoy 268 magic damage from Thunderlord's Decree

Many of champions make better usage of lethality builds such as since their comes as burst.

TL;DR Lethality isn't OP or broken, start building armor.