The easiest way to beat the weapons:

Have Gravity+Quadra Magic and 7x Counter+Mime on one character; In the battle, use Demi3 and you probably won't have to do anything else for the whole fight. You counter-attack when hit with 28x Demi3: 7 * 4 * ~40,000 = 1120000HP of damage in one turn.

Of course, Demi3 won't kill them, but they will take them down very low. One KOTR should finish them off.