Best way to live early game is to not die. /s
For real though, buying a defensive component can often turn a 100-0 combo into a 100-50 trade.
Taking zed as an example, going tear into armguard into RoA delays your first item massively, but lets you stay alive long enough to begin to scale. Going straight tear-RoA gives health, but you'll still be popped when he ults.
Zed at level 6 with 1 item should have 25 lethality, reducing armor by 18.33. Kass at level 6 has 30 armor at base and 36 with 1 armor rune. Assuming you have catalyst, tear, and a wand, you have about 1150 health. After applying lethality, you have 1353 effective health.
In the same setup, but going tear->armguard->ruby, you have 1080 health and 66 armor. After lethality, you have about 1595 effective health.
At level 6, zed's burst if everything lands on one target is about 1137 damage. The difference between 220 health remaining and 460 health remaining may not seem like much, but 1 Q is about 217 damage, meaning any poke before or after the ult is death (when combined with ignite).
Still, you're not going to "win" early game. It's a bit like playing kayle into the same match ups. Your job is to survive. That's it. You don't die, you farm where you can, you scale, then you hard stomp late.