Connect the Phoenix Summon and Final Attack materia in any linked slot. When your party dies, there Hitpoints will be completely restored. This makes killing Ruby, Emerald, and Sephiroth simple, just make sure you have enough Mindpoints to use the Phoenix Summon.

For more punch, add a Final Attack/Knight of the Round Table to the same weapon. Then when you die you'll resurrect and hit back HARD.

NOTE: This method is limited because you can only use Phoenix materia 5 times.