The best boss stomper build I've found is:
2 - Hu-Ben gatling guns
2 - Stun Needle Launchers (or Songbirds if the Needle Launcher isn't available)
BST-G2 booster
Talbot FCS
Then select frame parts to meet requirements and try and keep boost speed above 320
It's honestly a kind of cheese build that I'm ashamed to use. You just circle around the boss and blast them with your gatling guns until they stagger and fire the Needle Launchers which do a crazy amount of damage.
Like I expect they will nerf the Needle Launcher in the near future, it makes a lot of bosses trivial.
Of course the real, honorable warriors go into battle with a shotgun, laser sword and a pulse buckler. But sometimes you just want to move on with the game.
2-Ransetsu and 2-8 cell missile launchers
Just need good finger muscles to keep firing your guns
Me and my unswole trigger fingers