My first piece of advice is that the "
stalwart"
Beethro is protecting is not really a stalwart. It is just an NPC that is heading for the south exit of the room. It will not help Beethro kill the guards, and might even kill Beethro, if he's standing in the wrong place.
Therefore, your strategy should be to lead the guards to the side of the room, out of the way.
Click here to view the secret text
×Try heading towards the southwest corner, and turning north when the guards have safely passed the "stalwart".