The most dangerous part of {Possess} was its ability to control players. Players who are controlled by the boss would be more dangerous than the boss. Therefore, Zhang Yang and his gang rather kept their HP down to danger level of 10% remaining rather than become possessed by the boss.
For normal players, even if the boss did not have {Possess}, the {Shadow Trap} and {Shadow Roller} were already extremely difficult to deal with. While they are dodging either one of those attacks, they might get hit by the other. Each attack could deal up to 300,000 damage to 400,000 damage! So, how many players in the game could afford to sustain so much damage over time, then?
However, for high Tier players, the boss would be easy to slay, if the boss did not have the Skill {Possess}.
All they needed to do would be dodge, hit, and dodge and hit again!
Upon getting used to the pace of the battle, the boss would no longer be a threat to them