Whenever somebody starts up with the whole "Revive Aeris/th with a Phoenix Down" down bit, I generally like to refer to SPOILER the scene of Galuf's death in Final Fantasy V:

Galuf: Butz, Lenna, Faris, And Cara. Defeat... Defeat... Ex... death...
Cara: Grandpa! Please don't die!!
Butz: Galuf, don't die! Cure3!
Lenna: Please. Life!!
Faris: Wake up, ye landlubber! Phoenix Down! Elixir!

"Galuf's flame flickered out."

Cara: Grandpa!! Noooo! Don't die!!



If that doesn't do it, I don't know what does. Phoenix Down doesn't do the trick, and here's the proof --- they tried it, and a whole bunch of other stuff too.