To get this achievement you need to play as MASTER Shadowborn leader. What that means is that you have to either pick a character or crate your own character which have both Shadowborn Adept and Shadowborn Master aspects. Then you need to research bunch of strategic spells in order for Summon Fallen Angel spell to appear as it is tier VI strategic spell. When you research the spell, open your spell book on strategic map (not the battle one) and select it. Wait few turns till it will be ready to cast. When it will be ready just cast it on any army which have a free slot.