The Emerald Weapon is the easier of the two American Weapon. This is because the Emerald does not make whirlsand your charaters away. You need to need to have Final Attack equiped here because of the Emeralds biggest attack that takes off 9,999 HP off of all your characters, link the Final Attack with Phonix summon (this will revive all your characters at once). If Final Attack is level 5 (mastered) it will bring you back 5 times, which is a gift from God himself. As soon as the battle starts attack the Emerald with a Knights of the Round, then have each one of your charaters mime it. This will start a continous flow of Knights of the Round. You will be kill in the process a few times, also if your limit builds up do NOT use it, it will screw everything up. It takes about TEN Knights of the Round to kill the Emerald, this takes a long time and get really boring, but once you get the item the Emerald gives you, it'll make it all worth it. Just take the item to the traveller in Kalm (he is on the upper floor of the house in the eastern part), and he will give you comand yellow, red and green materia, which gives you all the summons, spells and comands (i.e. steal, fourslash, etc.)! For info on getting rid of the timer, see the footer text! GOOD LUCK!
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HINT: you have to get rid of the timer! To do this just go to the Shira under water plant and morph a monster called the ghostship, he is in the glass hallways, trust me you will know it is him when you see him!