The best advice to survive an encounter with a WEAPON: run away and hide. These nasty creatures were created by the planet to protect it from a horrible monster dubbed "The Calamity from the Skies" several centuries ago. However, the WEAPONs were never used because the monster was imprisoned by the last of the Cetra. There are four WEAPONs: Diamond, Emerald, Ruby and the Ultimate WEAPON. When Sephiroth starts using his black magic, however, the Calamity is revived is revived (and is a monster that you know all too well.) The Planet releases the WEAPONs, which go on a rampage seaking to destroy all that would threaten the Planet, be it the monster or Humanity itself. They quickly cause massive damage, and Shinra builds massive war machines like the Sister Ray and the Proud Clod to defend against them.

The Planet engineered the WEAPONs to cover all parts of the Earth, with Diamond dominating land, Emerald the sea, Ruby travels under the ground, particularly in the desert, and Ultimate wreaking havoc in the Air. If you really want to fight these guys (the only one you HAVE to kill is Diamond, during the game) I would recommend you power up and be prepared as suggested in the "Beating the WEAPONs" portion of this site. Good luck.