Plutonia is just outside the gate to Eden. When you kill her you get this achievement. It's as usual recommended to remove as many enemies as possible before taking on her. You can take out the pyro, robomedic and MIMIR Z600 one by one. Leaving only a hunter and a Tank and Plutonia for the last fight.
Since this is the last fight you need to go you can make it cheap by using a smoke grenade to cover your team at the end of the first turn and from inside the smoke cover hand grenade the enemies to death.
Or you can just fight the battle the normal way, she isn't that special especially not if you mind control the tank to fight on your side.