This achievement can be earned by completing the 4th main story mission titled "Best Minion Ever." You can't miss it if you intend to play through the story at any length.
There are two main boss fights in this mission: Boom and Bewm, and Captain Flynt. They are both relatively easy if you take your time and keep moving.
Normal Mode: Lvl 5
TVH Mode: Lvl 35
UVH Mode: Lvl 50