The first five minutes of the battle were very relaxed; Magistrate Barthilas didn't do much and lost a large amount of HP.
However, after this, it was like he suddenly woke up. He released several area of effect attacks and all of the melee players lost a significant amount of HP. Some players who were already low almost died and even Lu Li needed to retreat to regenerate.
When a level 50 Boss used a skill, it was like a level 40 Boss' Ultimate skill.
Lu Li was level 47 and the Boss was level 50. There was a 3-level gap, which meant that there would be a penalty applied. In Instance Dungeons, this affected a lot of things like Attack, Defense, Evasion, and most importantly, Critical Hits.
Otherwise, why would the early explorers of new maps not just take all the First Clears of the Bosses?