In horde mode, most players will choose either the church or the balcony-overlook as their starting command post due to the weapon spawn and abundance of cover and upgrades at both.
Due to the openness of the map, Mercy tends to be one of the more difficult Beast mode maps, but should be no trouble for a coordinated team.
The Easter Egg in Mercy can be triggered once you hear bell chimes from the church. While the bells are ringing, a picture of Dom & Maria can be found where the Boomshot\Digger launcher are. Shooting it will trigger an audio clip of Dom crying.
One of the most common strategies used on this map is too wait it out in the church area, and tag Frags up around the two entrances. This can be great for your team and also bad, as the enemy team can move the Mortar into range and use the exposed ceiling to shell down on members of your team.
Another strategy would be to use the Mortar spot as an overlook point, with some members protecting the entrances. This gives the overwatches plenty of area to cover, and haress the enemy team, however, the enemy team does have an advantage of getting the grenades and throwing them up to your team, causing chaos and death. Plus, the enemy team can use the left and right sides to keep you pinned.