SMITE

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.

As Morrigan transform into an enemy God and kill the God you transformed into.

Morrigans ult allows her to transform into any god in the game and use their abilities, basic attacks and so on. The goal here is to pick somebody that deals high damage such as late game hunters and high damage mages. You can play Arena or Clash since their are nine other people to pick from. However, playing these game modes can make it difficult to kill the target you want since theirs more enemies. So I would play Joust and make sure you pick a god that's on the other team and hunt that god down and the achievement is yours. Gods on the other team are in the red zone when you are ulting. Good Luck!