The battles get progressively harder, every 10 levels. Up to lvl 80 is usually fine to do, but 80-100 is hard and 100-120 is very hard. You will need a strong setup to do the end! You also need to have the monster +9 to unlock the last 20 levels and you need to build a team with monsters of the same type (4, 6, 8 respectively).
Overall, it’s a place in PvE to spend your tickets to earn gems and other rewards. It’s the end-game PvE content that is practically endless.
This is the thread EBO mentioned: KD's Player Icon previews