I suggest a nerf/rework to the atma's impaler item. I think instead of adding +dmg to a champions base attack it should on attack, do the extra +dmg as an additonal hit. In other words, a champion would hit with their base attack doing i.e. 60 damage, then right after atma's impaler would hit for another i.e. 80. Changing this would prevent what I believe are exploitable item specs on characters like Katarina. Going 4 Warmog's Armor, Atma's Impaler, and any form of Boots allows her to A) not die because she has so much life, B) kill everyone because atma's impaler gives her 200+ attack based on her HP C) take special advantage of her ultimate ability Death Lotus which scales significantly with attack damage.
In essence, not only does she become the best tank via shunpo dodge/6000 hp but also the best DPS as well. Limiting atma's impaler to a second-hit effect I think would prevent such drastic scaling on her ability and balance the game. Not only that, but also the damage could be considerd magic within the rework, allowing players with magic resist an opportunity to reduce the effectiveness of atma's impaler as a strategy. As it stands, it is a pure physical buff thus armor is the only thing that negates it. However, Katarina converts all that physical damage into magical instead, how can a player stack both Armor and Magic resist enough to counter both those devastating attacks? Anyway, that is my rant about atma's, hopefully my suggestion is taken into consideration.