The World boss fights can only be accessed via special sorties, you can fill up your gauge by completing 6 standard sorties and be victorious on them.
The other method to fight Bosses is after a season, there is a 1-hour time period where all bosses are present and can be fought.
You can tell whitch boss you are fighting by looking at the name of the map:
Ruins King (15G), GALEF PLANT - North Frontier
If you are looking for a certain Boss, you can select a faction that you want to fight, there is 8 boss in total each one in the different location on world map.
In this map you must fight 6 robots that you met already in story but now with much more AP, Predators are weak to KE, Hunters and Scavenger CE weak, so i suggest get at the top of the building as fast as possible, shoot them from top with heat machineguns and rifles+heat rockets. Heavy bipedal is the best for it.
Below is the video how this fight looks like.