Normal Bosses
Normal bosses each have 9 cards available to unlock. Defeating a boss unlocks one card, progressing from Bronze to Silver to Gold every 3 wins. Defeating the boss once also unlocks the next boss of the area. Each set of three wins improves the bosses level, increasing energy required, card level, boss health and combo play. The number of combo points available to the boss does not increase. Each boss in Little Town upgrades cards by 1 attack and 1 defense per level, or in other words, in the way of a bronze card.
Added in 1.25.3, each boss now has an additional two tiers of difficulty, Diamond and Onyx. Each requires three wins to clear. Currently all Diamond tiers give a chance at a Gold Upgrade, while all Onyx tiers unlock the Boss Card. There are no additional avatars for defeating additional tiers of difficulty. When progressing to Diamond and Onyx difficulty, the deck level does not increase, but health and energy required does. Onyx also allows the boss to use Fusion abilities with its cards, regardless of level.
Each sucessive area has harder bosses which also increases energy, health, card level but also combo points available to the boss. Once you have defeated all bosses in an area at least once, the next area will unlock.
The "deck" a boss uses is also not fixed. Most (probably all) contain the nine cards dropped by the boss, although they may not all be played in a normal game eg Bronze cards. Each boss will play ~15 unique cards, and will play approximately 45 cards worth of plays, but the exact number of each card, whether it is a Combo or Final form and card order is randomized each time. This allows for actual card counts between 39 and 45 cards, so the final determination may be a set number of turns. It is then likely that the cards allowed to be played are random, so that the AI "luck" matches the lucky draw of players.
Added in 1.27.0, the bosses of Little Isles upgrade card level by +2/+2 per level as you progress. All bosses in the Enchanted Caverns being at Level 4 with +3/+3, but all bosses of the Coven have Level 4 cards with a +6/+6 bonus instead. Beginning at Silver difficulty, the Enchanted Cavern bosses get Level 5 cards with +8/+8, making the difficuly between the difficulties much higher than normal. Not only this, but the new bosses lack the ability to obtain diamond rank or onyx as of 1.27.0, though you still can unlock diamond and onyx reward cards and will play diamond cards after you complete the gold tier. These bosses are the first to incorporate the new Void card, a silver combo card, which unlocks new combos and new final cards. Prehistoric and Knowledge are the other new Combo Cards , although no new events appear to have been added to provide them.
Avatars
Defeating a boss 9 times unlocks a "Gold Mastery" avatar of that boss. Added in 1.26.1 defeating a boss 15 times unlocks "Onyx Mastery", which is the same avatar with a gold border.
Defeating all bosses of Little Town three times unlocks a Chinchilla avatar. Defeating them all 6 times unlocks a Firefox . Defeating them all 9 times unlocks Snakes on a Jet . Defeating all bosses 12 times unlocks the Emok avatar. Defeating all bosses 15 times unlocks the Pastry Cat avatar.
Defeating all bosses of the Little Isles three times unlocks a Shark avatar, six times unlocks a Brachiosaur, nine times unlocks a Triceratops, twelve times unlocks a Saberchilla and fifteen times unlocks a Tyrannosaurus (Prehistoric). All bosses here also list two avatars unlocked once Gold Mastery is completed as of 1.27.0. This will probably be changed in later updates to Onyx Mastery for the gold avatar similar to the previous bosses.
Term Overuse
Chloe is a boss but her story mentions the boss which is Peter.
Gus is a boss but his story mentions the boss which is Gale.
The stories for Elmoira, Clyde, and Dr. Robo refer to minions.
The story for Nessa mentions the Dark Alchemist.
