This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
In the Japanese version of FF7, all you had to do was attack. In the US
version, these guys are invincible to all forms of damage unless you use
an Elixir on them first, which will nullify all their defenses. Need
Elixirs? Try stealing them from the Master Tonberries that roam the
lower levels of the final dungeon. If you're fighting more than one
Magic Pot, try using a Megalixir to affect them all (that is, if you
really want to waste it).