With full offensive masteries & ignite, she becomes an unstoppable squishy killing machine at lvl6 (like 40secs after hitting lvl6)
The counter? BUY HP! An Akali will go for the lowest hp opponent, all the time, in every fight. Try not to stand out of the crowd having 2k hp at lvl18.
If you're playing a squishy and she hits lvl6, don't think that your lanemate will save you from the kamasofdoomz, because she just ults to you, shrouds, ignites, goes mark+hit+crescent+ult+mark+hit and you're dead, and she can ult back to your front row creeps before your lanemate can say wtf.
Don't think that you're safe chilling behind your caster minions with 70%hp as Sqishyplank; she will eat you alive.
The way she rolls is "I'll just wait stealthed in the middle of a teamfight until someone gets fairly low (or there's a Veigar with 1800hp) and I'll unleash the kamasofdoomz on him; rinserepeat."
BUY GOD**** ORACLES, as they shut her down hard. Your tank should get them, as Akali barely scratches tanks.
A bit of MR, lots of HP, and Akali won't even think of attacking you, and even if she does, her damage will tickle.
If she got a couple of kills early, get oracles, rape her.
Most people do NOT get oracles vs Akali, which is a huge mistake when she's running around with 20 Mejai stacks.
Still, if you're running a tanky team, you'll just laugh at a 20-stack Akali trying to do dmg.
tldr: go tanky.