You don't have 9 people to choose from as KriSpy has put in his guide, you have 5, as you need to pick an enemy god and kill them before you transform back into the Morrigan. Depending on the Gods picked in your game (Arena would be a good mode for this), you may have a hard time. People with heavy hitting/long distance ult's are always a good pick, such as Thanatos, Poseidon, Kulkukan etc.
Another thing to do to try and give you the edge over this person when you become them, is to grab the damage/attack speed camp before transforming (depending on the God), to give you an edge if they don't have a buff themselves.