(i´m portuguese, so my english is not perfect)
1. With Gloria in your party, go to her cauldron in Canonia. There is the recipe for the Zombie Juice.
2. Then go to Saul´s house, and fight some monsters until you get the ingredients.
You´ll also need bat wings (try the Gem Mines to get them fighting leech bats).
3. When the battle with World Saviours comes in Fire Temple, turn at least 3 of them in Zombies dropping the Zombie Juice on them.
4. They will atack them selves until they all die.