Eliminating Unsuspecting Prey
Looking at the description for the puzzle, it seems like they had to read out runes while a designated person would enter in the password.
The way this works is that there is a central platform and four rooms. Each of the rooms will have a small wave of monsters and one of the adventurers would need to protect someone while they read out the rune and its position in the password.
Once all four runes are collected in the right order, they'll be allowed into the next level of this stage.
The slayer would need to either A, delay the situation and let the monsters eliminate the people or B, they do it themselves.
Since they're moving in pairs, the slayer would be revealed should they sabotage too obviously. This also forces them to be the ones that have to think about what they're doing in order to pass successfully. Then again, the reward for succeeding is a free pass to the next round so the task must be hard.