Akali is actually fairly weak, at least at high elos. Akali has been nerfed into oblivion because she has a problematic kit. A lot of this is due to the binary nature of her shroud. Early game, before enemies have pink wards, the shroud is quite strong. Late game, the shroud is pretty much worthless because any decent team will plop a pink ward on top of it. In addition, her 'e' is an energy sink that does not synergize with the rest of her kit (after it no longer proc'd her mark of the assassin). This means that she is basically just damage on a point-and-click q, a point-and-click dash, and damage on her dash. This is incredibly difficult to balance, so if she is viable at high elo's then she will be a pub-stomper at low elos.
Her entire kit needs to be reworked. In particular, her shroud cannot be hard countered by pinks and she needs to be forced to utilize her shroud late game. If done correctly, this will allow her damage to be nerfed, allow for more skill and counterplay, and generally make her easier to balance.
Here's a recommendation for how to rework some of Akali's skills:
http://boards.na.leagueoflegends.com/en/c/gameplay-balance/A5uRzwK7-akali-assassin-update-recommendation