Losing Control
With the current ability of his mage profession, it was definitely impossible for it to be able to deal with the high level monsters available within this level of the dungeon.
But even then, there was one thing that Henry was completely exploiting during the time that he was here. That was him utilizing the mage profession, trying to immobilize the monsters within this dungeon, before killing them off using another skill of his.
With the current power level that he possessed, it was obvious that he could simply kill the monsters that were still within the diamond rank with a flick of his hand. But even then, he was not doing that, giving the mage profession a chance.
But even then, since the level of the mage profession was extremely low, it could not even have much of an impact on the monsters. The only effect that it had on them was to make them a little surprised considering that they realized that their movements were being restricted.