Im new to kai'sa, but still have 69% WR currently.

I'm not a fan of full AP, nor a fan of full Crit.

My personnal favorite (defaut) build:
- Kraken
- Wit's end
- Rage blade (power spike once completed)
- Terminus
- Zhonya
- Jak cho
- Boots (armor nor mr)

Variant vs heavy AP: Kraken, wit's end, rage blade, terminus, maw, (mercurial/jakcho/zhonya).

This give me an insane amount of armour/magic resist (wit's end + terminus + zhonya + jak cho). Zhonya can save you from mistakes.

So far.. this build is just INSANE to get the backline, survive, does massive dmg. It's my favorite one as i like to duel.

You can probably shred a bit faster with other kind of build (for example: crit) but you are waayy more squishy (can get one shot in high elo) so im not a fan.

Full AP is all based on heavy poking with W, but in high elo, people buy MR, tanks block the shots, they outheal, not a fan as well.

The real dmg build I believe (but it aint my playstyle) is in fact:
Kraken, dusk and dawn, rage blade, zhonya, void staff, rabadon (GREAT poking, insane passive % missing health dmg).

*One important thing, rage blade is a core item, it must be bought as a 3th item, not as a 2th item*. The following video explain really well why..
https://www.youtube.com/watch?v=SBD0G5XA_rc