Tifa’s weapon, Premium Heart can be obtained from the Item Terminal in the Wall Market, which is located in the Midgar Slums. In order to be able to get back into Midgar, you must head to Bone Village and dig for the “Key to Sector 5”. Dig on the upper level between the campfire and the smoke (this may take a few tries) and once you got the key head to Midgar. Go to the Wall Market and head to the item terminal (remember the one that shoots at you?). Once you press [OK] on the terminal, you will be handed the Premium Heart.
To do max damage with this weapon, make sure Tifa’s limit break is full. Of course you can’t do normal attacks if limit bar is completely full but there is a way around this. The best way is either to equip, 4x cut, deathblow – or alternatively, equip a number of counter attack materia’s and select defend on Tifa’s turn. This will ensure that she will do max damage with this weapon.