To get this achievement you muse play a Skirmish game as a MASTER Keeper of Peace Leader. The spell to summon Arch Angel should pop up after researching few things. When it pops then cast a spell on one of your army (it can't be full) and you will get the achievement.
To those who don't know what Master of the Keepers of Peace means. Basically when you are creating a hero (or picking one but I'm not sure if there are any masters of it) when you have to pick 3 aspects of your hero pick Keeper of Peace which will unlock second tier of Keeper of Peace which you need to select.