Tifas limit can you get after Cloud is back from his Lifestream sequence on "Disc 2".
Go with Cloud and Tifa in the Team to Nibelheim.
In Tifas room in her house, it is the house on the right side in the middle, is a piano. Just play and hit those buttons.
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+
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+
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and then
After this is done you find in the piano Tifas Final Heaven.
Like all other characters finish all other Limits and use the item. Achievement done.