Tifa's weapon, the powersoul deals more damage when near-death, but she also deals more damage if she's under Death Sentence. It works like this:
Critical (yellow) HP: 2x damage
Enemy Skill: Death Sentence: 4x damage
Materia: Deathblow: 2x damage
Together, that's 16 times normal damage. Deathblow also ignores back row, so it's 32 times back row damage. Awesome, huh? And in addition, we equip her with armor and accessories to boost her strength. This is what makes Tifa the ultimate killing machine.