Why don't people play Katarina a bit tanky? She'll get completely focused on when she does her ult, which has pretty good damage anyway. I'm imagining the complete build to be like this:
- Sorc Shoes
- Atma
- Warmog
- Maw
- Randuin
- Rylai (extra slow) / Zhonya (wait out the cooldown a bit) / Banshee (might save from CC)
Core is probably Sorc Shoes, Atma, Randuin... reeeally pricey.
Preferably with big teammates like Alistar, Naut or Cho so it'd be harder to target her... plus they have hard CCs to prevent enemies from CCing her, specially Cho with his 3 second AoE silence.
I don't know what "people" you're referring to since I already do that (that is, build tanky items on kat rather than pure AP). =P
Ideally you get amumu and wukong on your team so the triple ult combo is ridiculous. You have to bear in mind that you can still get killed by ulting into amumu's ult because people will nuke you while snared unless they're just confused.
I heard Maw is good on Katarina but I've probably only built it once or twice, and haven't worked it into the build yet. I use armor yellow and magic resist runes to be defensive to begin with, and I've started usually building mercury treads instead of sorc shoes if I'm against an AP mid with CC or heavy magic damage (which is most of them), unless I'm just totally dominating and the other team doesn't have much AP.
I try to go for gunblade (at least the spellvamp part) for sustain, then in -theory- hourglass for armor + invincibility, but I probably end up getting abyssal scepter (moar magic resist) a lot of the time.
Especially with the nerfed atma's, I wouldn't imagine atma's scaling (you don't need the crits either) would be as nice as AP scaling. Rylais can be nice for HP and extra slow on top of gunblade. I was actually considering randuins last night, but I don't usually find myself needing that much armor. I was also considering banshee's since the spellshield during ult sounds like a good idea, but of course the mana part of catalyst is totally wasted.
Pricey items are not as big of a deal on kat as for other champions, since I tend to go something like 8/2 or 16/4 if I'm doing well in games, and probably end up the #1 or #2 person for gold on my team.
I've built mejais a couple of times, but at terrible elo I've had a lot of success with the above strategy, gaining maybe 230 elo so far. Could be that I've reached my peak though, and I assume I will start getting completely countered at some point, but I still also don't feel like I have reached my maximum potential playing her.
Side note, I think the main guide I was following mentioned tank masteries (9/21/0 or whatever) as possibly being good on her, but I haven't tried that yet.