Minibosses are special enemys you can fight through events.
T.O.R.V.A.K. Miniboss[]
When you arrive at the first quest marker for the Sonic Screws the building will either have been destroyed by Raiders or it will be fine,
If it's been destroyed chase after the Raiders, then at the second quest marker kill the Raiders and you should find a location for a mobile construction facility.
If it's fine radio them, ask about the sonic screws, then they will tell you that they have a mobile construction platform so they give you the rendevouz location.
From then on it will continue the same way.
When you arrive at the next quest marker ask to buy the sonic screws, they should tell you that they are not for sale, then attack them and the boss fight will begin.
The miniboss is a Prototype SRS Command Vehicle.
Beating it will unlock the Techno Troubles Achievement, it will also unlock the SRS Mobile Command Vehicle.
Info on how to find it from Here.
Raider Miniboss[]
To find the Raider miniboss you need need to have the Blue R998X Chip quest, doing that quest will ALWAYS lead to the Raider miniboss.
The miniboss is a Dreadnaught Mk2.
Beating it will unlock the Roughneck Rampage achievement, it will also unlock the Dreadnaught.
Privateer Miniboss[]
To find the Privateer miniboss you need to have the Multicellulose Infused Ethanol main quest.
At the chemists lab you shoot the chemist and read his notes, there should be something about a Driving Dutchman, then go to the next marker and you should find the miniboss.
The miniboss is a Cargo-14 A2.
Beating it will unlock the Plundering Pirates Achievement, it will also unlock the Cargo-14.