Once you have the Boss Horn, and upgraded it to level 2 (you will need Town Hall level 6 for this upgrade), you can summon a boss for a boss battle. When you Boss Horn is level 2 you will see the summon boss symbol at the top right of the game screen. Tap it top start the boss battle process.
In a boss battle all of your hunters get involved to try to defeat the summoned boss within the set time period.
. During the boss battle your hunters can die and go back to the town and restore themselves and return to the battle to keep up the fight.After the boss battle there is a 20 minute cooldown period as your horn needs to assemble before you can blow it again! The Boss Battles are really difficult and you will need a high level and well equipped team to defeat any boss / even the first one.
You can however go back Difficulty Levels to try easier bosses which may work out better for you if you are struggling. Alternatively you can wait a while, upgrade your hunters and their weapons and armor and retry at a later date.