Why do people always say that? I always read posts of people saying Akali's main source of damage is her ultimate. It's not.
I am going to tell you how to play against Akali.
The majority of her damage is in one spell, her Q. Her Q is the spell that is capable of dealing 40% - 60% of a carry's health, especially if she has Sheen or Lich Bane. That is only if she procs Q her with an auto attack or E. Everyone thinks it's her spammable ultimate that does all the damage. That's wrong, but I will say that she needs to be able to have that ultimate because once her Q goes on cooldown she's at her most vulnerable because she stops dealing damage, therefore stops spell vamping. Her damage drops drastically if she no longer has any charges on her ult. So making her waste charges on other champions or minions to catch up helps a lot. If she has a Sheen or Lichbane she is capable of dishing out some decent damage if she procs them with her E or R. So know that her sustained damage is not great if she does not have Sheen or Lichbane. Her W is vital to her staying alive but is easily countered since everyone is capable of revealing stealth at level 9. Not to mention you can just buy vision wards.
She already has a lot of counter play. CC demolishes her, her stealth is easily countered, and you just have to dodge her until her mark is removed from you to counter or delay her high burst. A defensive support helps with that and she is very squishy. She falls off late game anyways because she can't fight 5v1. Sometimes all it takes to counter her is both the AD and AP carries to hang out with each other late game. Keep sweeper or vision ward on you. Banshees on the AD and zhonyas on the AP. She won't be able to do enough damage to kill both. Those defensive items go a long way when fighting Akali so get those items mid game because that is when she carries her team the most. They should be the third item, maybe second to finish. As I stated earlier, her Q is where most of her damage is at. If banshees absorbs the mark she won't do that high burst. If she uses ultimate or E to break Banshees, her sustained damage is gone or she is down one stack making escape more plausible. Zhonyas is obviously really helpful because she needs to use at least 2 stacks of her ultimate to kill a carry and one full rotation to kill a carry late game. If you use it before she can finish her rotation her cooldowns will be out of sync and again, her sustained damage diminishes after her initial burst. She will probably have to change targets and she will be lacking the burst to kill and the spell vamp to survive.
If you're laning against her just buy a vision ward on your second back. Once she hits 6 make sure the jungle comes to gank her. After about level 8 and she hasn't picked up any kills she won't do anything for the rest of the game and you are home free if you stick together as 3 or 5 late game.