Either side of the south end of the mesmer trench, there is a patrol, and in the middle is the mesmer lord, behind an earth, heart, sanity, flesh, and spirit tormentor. The left patrol is a mixed group of 8 Stygians and Tormentors. The right patrol consists of 2 Tormentors and 6 hungers. Depending on the timing of the two patrols, you will grab one side first, followed by the lord, then the other patrol. Pulling the left side first is the most common, especially if your team has a monk with EoE. Because the groups include Tormentors, you cannot use Dark Escape while pulling.
Walling is done by the Emo or the Main Tank. This depends on preference and whether the tank is able to get in place for it. Even if it's the MT's job in your tactics, the emo still needs to be ready to wall in case they are unable to get in position.