Press the back/select button, click right on d-pad to missions, scroll down on it until you see Legion Occupation and under it should be "GOLEM", go to the task and defeat it. Their is no level or any kind of requirements for it, just luck when the mission pops up.
TIP: I did it right after I beat the main game, and the mission was available right away. Hopefully this helps if anyone is still having trouble with this.
Video Guide:
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