A Colossal bomb is an item used in the Dominion Tower. They can be received as a power-up reward after all of the journal pages have been found in addition to a Dominion Factor of 10,000. They are also available as a power-up in the Special mode match Hey! Eat Power-up.
The bombs can be detonated in a Dominion tower battle to deal a large amount of damage to all enemies in the arena.
Once you have unlocked the power-up on the Tower's rewards screen, you can receive colossal bombs as rewards even if you have a Dominion Factor lower than 10,000. For example, it is still possible to receive one after completing a single boss fight in Endurance mode and quitting.