Boss battles are generally seen at the end of a stage or level, so you must kill it in order to progress and unlock the checkpoint an the next stage, the bosses are generally far stronger than the opponents the player has faced up to that point. The boss enemies are larger in size than other enemies and the player character. Commonly, bosses are very hard or impossible to defeat without knowing the correct fighting approach. Bosses take strategy and special knowledge to defeat, such as having specifics elemental damage or elemental resistences to attack where they are more vulnerable.You will encounter a boss every 20 Floors, as the name "boss" entails its a tough monster you will need to be prepared for the battle before hand, making sure you have Hp potions, Mp potions and the best equipment you can at floor segment.
Stage 21 BOSS- Stone Golem.
Stage 41 BOSS- Huge tunnel worm.
Stage 61 BOSS- Kraken
Stage 81 BOSS- Satan
Stage 100 BOSS- Nameless.