The branches of the trees of the Serevian Dungeon had incredible penetrative power. Rui recalled when he had first entered the Serevian Dungeon, he had been penetrated by a branch that easily impaled him with no difficulty, despite the Adamant Reforging technique making him extremely tough and durable. It had also moved fast enough to bypass his reflexes when he wasn't alert and focused.
This meant that it was extremely likely to be quite lethal to most Martial Apprentices. However, this never mattered because the branches weren't a threat to Martial Apprentices they were only a threat to threats perceived to be in the Squire Realm.
This was the very core of the plan that Rui had come up with. If he used the Squire-level mask, he would be triggering the tree and the branches would be unleashed toward him.