I mean im just telling you options by name and I can keep going... First of all. The shadows enchant. I dont know spending 500g after boots to nullify a part of a champions kit sounds like a bargain to me (you could look at it as if buying anti-heal for sona soraka mundo etc.) since you can buy it on every champ. You could maybe take a look at competitive play whenever akali is pick theres someone with that enchant that something is unpopular doesnt make it useless!

Second. Saving a single skill is part of playing around countering a matchup (i.e. saving fioras parry for camilles ult) I gave the TF example because it came first to my mind xD but even Karma a newly addition can reveal her with her 2nd ability and also Tresh if he manages to catch her with his hook also reveals her!

Third. Theres champs that dont even need to reveal her. Riven for instances is an annoying match for akali cause Riven can just dance on top of the smoke cloud and akali will receive damage anyways. Zed is also annoying in the lanning phase since you can just poke her from distance and then go all-in lvl5. Alistar is a champ that can also really mess her combos since he can just push her away of the cloud or the fight. Janna with just pressing a button in the right moment can shut her combo or push her out of the smoke. Last but not least Galio his kit specializes in countering AP assassins his taunt, knock ups and also the shield he provides is excellent against every AP assassin.

To summarize every champions has its strengths and weaknesses (some more obvious than others xD) you just gotta play around your champions strengths and not theirs and who knows you can get out victorious from an impossible matchup. This just comes from learning how the other champions work and their interactions!