Bosses are opponents, that can be fought by a group of players from your friends list or from
Spirit sailor

Spirit Sailor boss. The level 1 boss for Spirit Gem raids

the same galaxies.

Bosses can be summoned from Assemble Team menu, if player's inventory contains appropriate key.

Keys can be assembled from fragments acquired by fighting storyline missions from War menu or by visiting friends. Type of key fragments depends on current chapter:

Key Chapters
Bubble Gem 6 and 7
Spirit Gem 8 and 9
Demon Gem N, 10 and 11
Metal World Gem 12 and 13
Key Boss name Army/Coupons Description
Bubble Gem lv.1 Bubble Kid 100M/105 This spoiled teenager loves visitors, which he use as practice target.
Bubble Gem lv.2 Bubble Queen 150M/105 Don't mess with the queen of bubbles, or she won't be very happy.
Bubble Gem lv.3 Bubble King 200M/239 The king of bubbles would like to teach you some respect.
Spirit Gem lv.1 Spirit Sailor 150M Don't be fooled by the goofy looks of this drunken spirit sailor.
Spirit Gem lv.2 Spirit Captain 225M The captain of the spirit fleet isn't too happy about your recent attacks.
Spirit Gem lv.3 Lord of Spirits 300M Your invasion has angered the lord of the spirit world. Be prepared to fight.
Demon Gem lv.1 Demon Guard 200M This elite demon guards the entrance to the demon world.
Demon Gem lv.2 Demon Lord 300M Lord of the demon world has been summoned to take care of the intruders.
Demon Gem lv.3 Aspyromuth 400M You have reached place of Aspyromuth, the king of demons. Will you be strong enough to survive?
Metal World Gem lv.1
Metal World Gem lv.2
Metal World Gem lv.3

Coupons are those awarded to the raiding team players on the basis of damage dealt.  Base coupons given to the Gem owner not yet included.