Tifa's Ultimate Weapon

Tifa's ultimate weapon is Premium Heart. It can only be found in Midgar, which requires you to get the key to Sector 7. To find it, walk into Midgar and talk to the man standing by the gate. He says that he might have dropped it at an excavation site. Now go to Bone Village. Talk to the staff leader and tell him you're looking for normal treasure. Place the staff around the top area. Dig where their points of vision meet and you should find the key. Use it to return to Wall Market in Midgar and check the broken vending machine. It should spit out Premium Heart.

Tifa's Level 4 Limit Break

The last part of Tifa's Limit Break combo is called Final Heaven. To get it, put her in your party and go to Nibelheim. Enter her old house and play the song from your flashback on the piano. The song is x, square, triangle, R1/L1 triangle, R1/L1 square, x, square, triangle, R1/L1 x, circle, x, square, x. Tifa should now find Final Heaven in the sheet music for that song.