When a teamfight breaks out, don't be the one to rush out in to the front lines, becouse that way you will get killed fast and won't be of much use :)

Best tactic for Akali is to not be with her team, but somewhere to the side, where if a teamfight breaks out you could instantly come. When a fight starts, wait a second, let bruisers think you're not there and maybe walk further away from their squishies, then Shadow Dance on the weakest enemy targets: the AP and AD carries, and quickly burst them with your combo so they don't damage your team too much.
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This also shows, that if your enemy team are not used to akali, you can get easy kills on them becouse they might not understand your damage potential, and focus someone else.