Through observation, Zhang Yue discovered a hidden rule in the Level 4 Gladiator Arena that wasn't explicitly stated.

That is: the monsters in the Level 4 Gladiator Arena are also categorized into levels 1-5, just like Life Containers.

Killing a level 1 monster allows the Life Container to absorb 1 point of life force, a level 2 monster provides 2 points of life force, and so on.

Using Scouting, one can view the monster's level along with the data.