Interference.

Rank 4 to 6 monsters are not found in nature. They are spawned specifically for a purpose or for a certain participant. The decision of what type of monster to spawn is taken after careful consideration and analysis of the strength and weaknesses of the participant in question.

This group of monsters is meant to ensure a 50/50 chance of death. You can still be killed by them even if you're stronger than them. There's a definite chance of death when fighting them. So it is important to always be attentive around such monsters even if you are stronger than them. 

This second group of monsters is not supposed to be introduced for at least one month. Even those that were designated as KINGS right from the start in each arena only get to face rank 3 monsters at first. But Soverick increased his threat level by breaking the shackles of the world on him immediately after the challenge began by flying.