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:
Wasteland King (15G), MAP NAME: FORT DENIS / INNER CITY - South Island
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, you met those in story mode, but now they got much more AP, the most dangerous ones that using boost charge are CE weak, so you can kill them fast by either heat cannons if you play a tank or heat machineguns+heat rockets if you play normal AC ( i suggerst using a heavy bipedal with decent TE defense), the other 4 are only using some TE guns, and are weak to KE weapons, so you can kill those with autocannons or rifles.
Below is the video how this fight looks.