Mythic
I just completed this achievement by watching a video on Youtube. I kept failing even though I completed all the mentioned steps. What I learned later was, both achievements cannot be done in the same run.
Explanation: The first one: Waiting for Gerdo, requires you to kill all five Sentries first. Then, you have to engage the boss and wait for him to call for backup Signal Beacon. Once the guards have appeared, you can either kite the Boss and Adds to all 5 beacons to shut them down depending on your character level/endurance or kill the adds right away and then drag the boss to each beacon. Side Tip: Gerdo will also have a count of how many beacons are still activated attached to his name tag.
Summary: You cannot complete both achievements in one run because the first requires the FIRST BOSS to be alive and the beacons to be shut down during his encounter and the second requires all five Sentries dead and the beacons to be shut down BEFORE engaging the FIRST BOSS.
Location of Sentries:
1st one is near the first beacon
2nd one is patrolling the docks to the left of the 1st Sentry area
3rd one is patrolling in the courtyard area right after and just behind the 1st one
4th one is standing in the doorway next to the 2nd beacon
5th one is patrolling the bridge with two mobs right behind the 2nd Beacon