Instructions
Play through the game until you reach experience level 50 or higher. Firebombs are level 51, and you need to be at least close to their experience level to have a chance of defeating them.
Go to the Cave Within a Cave at Hidden Street, the Fire Chamber at MesoGears or Spa M or F at Zipangu to find a Firebomb.
Fight the Firebomb with ice attacks and ice magic to defeat it. The Firebomb is also vulnerable to attacks using poison, holy, dark and lightning magic, but those attacks cause it only small amounts of damage. The Firebomb has 3,200 hit points, so it will take some time to wear it down. Never fight it with fire attacks, as these can actually heal the Firebomb.
Run away from the Firebomb when it starts to glow. Once the creature's hit points are under roughly 100, it will explode and cause damage to anybody near it. Alternatively, prevent the FIrebomb from exploding by attacking it continuously once it starts to glow; it will not explode if you take away all of its hit points before it ignites.