Oh look, another one of these.
You got shit on by an Akali, and are here to cry about it.
I've said it before and I'll say it again: a champion beating you after somebody fed them does not make them OP.
You made this post four minutes after a game ended in which you got absolutely demolished by an Akali, who was 18/3/12 due to a particularly incompetent Vladimir.
Even Soraka can be a nigh unstoppable hyper carry if she is that fed.
Moving on to the actual post...
A champion being all targeted doesn't mean they have "no counterplay". Her Q relies on her getting the follow up hit for most of its damage, which is easily abusable. Her R would be stronger if it were a skill shot; because it's targeted, she can only use it offensively in most situations.
(also, her W and E aren't targeted spells, so that's BS)
She has SOME sustain, sure, but it's nowhere near as exorbitant as you make it out to be. It's decent lane sustain in the laning phase, but it's only good for keeping yourself topped off since trying to use it while you're actually low is just going to get you killed. It's strong in-combat sustain late game, but by that point her sustain should be the least of your worries.
She's mobile, but can only really go in one direction. Her ult can't be used as an escape unless she's already balls deep and there are some enemy minions around.
She has invisibility, yes, and it's her only real defense. It's hard countered by true sight (Ignite, Soul Shackles, Oracles Extract), damage over time (TY Shimmer), and any large splash damage. The downtime is short if she gets a couple CDR items, it's still long enough that she's gonna get bursted.
Akali is very much a kitable champion. She has some dashes, sure, but she's not like RIven. Those dashes take a while to come back up; if she doesn't get resets, she's a sitting duck. Considering her ult can be interrupted by displacement effects... yeah she can have issues with target access.
If you think Akali is a stat check, why don't you try playing her? I mean, such a simple and OP champ must be free elo right XD.
Just because it's a common answer doesn't mean it's a wrong one. This game has a LOT of glass cannons; keep em from shooting and they'll shatter.
You don't have to one shot her, just protect your carry for a little while so they can kill her. As for the "Off-tank" thing, nope. That stopped being a thing a long time ago; the Akali in your game got a spirit Visage because your team was very AP heavy and she had enough mejais stacks that she didn't need any more damage. Most Akali's go for a pure damage build; the only defenses they get are Zhonyas and Banshees, which don't exactly tank them up.
Yes. Yes that's fair.