Tank Akali isn't OP
Tank Akali is a counterpick to champions such as 
top. She wins these lanes because it lets her get away with her garbage early game, and counters their good build paths. Since Akali has good base damage she can get away with building minimal AP if she has levels over her enemies. She has way less burst with this build, and thus way more counterplay if she gets ahead. Simply get your front line on her, and let them hardcore peel. She acts like a bruiser if she reaches your backline (if they reach it they'll be dead in a few seconds).
If you don't want her to get ahead play things such as 
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to get an easy lead. If you punish her early she'll fall into the pit of uselessness that Akali always falls into if she gets behind.
