The final boss is a massive version of the Keeper with 9 different parts, 2 weapons that look like pulse lasers, a mine layer, a missile launcher that will launch massive amounts of missiles all over the place, 2 laser beams that can't swivel, 2 shield generators shielding the mine layer and the missiles and finally, a bit the comes out of the front of the boss after everything else has been destroyed along with 2 gun turrets that can only be destroyed by destroying the entire boss.
The boss also summons Keeper drones.
Beating it unlocks the Made It Achievement, beating it on Medium and Hard unlocks the Medium and Hard versions of Made It.
Minibosses page.