The Fusion Weapon Lab is an Evil Symbol in Disgaea 4 that lets any monster in the Evil Area magichange while fused. This makes the weapon even more powerful and increases the range of skills. Using a fused fist makes the range of Big Bang exactly that of the monster formation in Mount Ordeal 4, making it ideal for farming. Another option for someone who only wants to use swords is to get a fused magichange sword, stand one step back from the first enemy, and use Rune Dimension, which takes out all but the two on the very sides.
You need to kill a level 100+ monster with a Fused one to unlock the bill.