It is optional to add mini boss to the game. If added, they need to be add for the entire campaign.
When you play with the mini boss, take a random mini boss from the specific temple. When 2 specials rooms are succesfully activated, put the mini boss in the enemy deck and shuffle the discard pile and the enemy deck together.
When the mini boss is revealed by engaging an enemy, the mini boss is put in the room it was engaged from and stays there until defeated (except if revealed from a special room, in that case, put it aside and draw another enemy card from the deck then put back the mini boss card in the enemy deck and shuffle the deck). A mini boss occupies ALL the space in the room.
Mini boss has a special permanent ability that affects the whole temple until defeated. As long as the mini boss is in play, the ability is effective.
When engaging the mini boss, all hero engaging the mini boss need to roll for damage. Then, they can try to defeat it. The reward depends on the level of the temple. Like the boss, you can try to defeat the mini boss by paying the defeat cost X times (but no more than the number of player) to receive the reward X times divided between the engaged heroes.
Items such as arrows can't trigger remotely the mini boss. And like bosses, a player can't assist a hero engaged to the mini boss if he is not in the same room.
When entering a room with a mini boss, you need to engage it. After rolling for damage, even if undefeated,the player can resume is action. Hero doesn't engage other enemy when entering the room since the hero need to engage the mini boss. But a hero can, if wanted, engage an enemy in his space to defeat it and get his reward such as items or activating a door or a cog.
The reward for defeating a mini boss is not kept after the temple if the players don't successfully complete the temple. Meaning, for level 2 and 3 mini boss, the piece of heart or the triforce shard gained, will be kept by the players ONLY if they beat the boss. In case of failure, the triforce shard or the piece of heart obtained (or the heart container if you already add 2 pieces) must be discarded (if a heart container is discarded,the player takes back his previous 2 pieces of heart). That prevents players to farm mini boss to get a good reward, then fail the temple intentionaly to get the reward again. In case of the level 1 mini boss, the reward is a time token. They are not kept for the next temple for any reason.
When a mini boss is defeated and the players beat the boss of the temple, put that mini boss aside. In the level 4, Ganon Tower, all mini boss collected this way, will be mixted in the enemy deck at the beggining of the game. They are considered level 3 mini boss, but they don't give any rewards to the players if defeated. They will just be there to hinder the players.