Minibosses are the most powerful recurring enemies in the game, they can take and dish out a lot more damage than regular enemies. They show up in certain missions and when you max out the noteriety for a faction (Marshall, The Idols, Los Panteros). They are indicated by a skull on the minimap. The latter two you just shoot until they die, but the Marshall miniboss first requires you wait until they tire, then shoot the protuding canisters, then finally shoot his back to deal damage.

You're guaranteed to get this through completion of the KAKTS Radio venture as you need to call each faction, raise their noteriety to max and kill a miniboss for completion.