Bosses will spawn just like other characters. Team up with other user from the same server and defeat it to earn fuel and XP for the character that you fought with. The higher the level of your character the more damage it will do, and the earlier you fight the more XP the character will earn.
You first need to equip a character using: im.activity equip [local id]
If you want to attack the boss, use: im.fight
Example: im.fight
It is most beneficial to fight with your character that has the highest level to defeat the boss the quickest.
If the boss is not defeated until a new boss spawnes it will not be able to be killed anymore. Your character will still get the XP from the fight, but there will be no fuel reward given.