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.
Shotgun/chainsaw/songbird on the lightest AC you can fit it all is also surprisingly good against a lot of things, and the pile bunker in place of the chainsaw usually works for the things the chainsaw doesn't work on (because the chainsaw does multiple interrupting hits that ramp up in damage, so any boss that doesn't get interrupted like that will usually just up and leave while you're mid charge attack).
For late game the coral generators are great for reverse joint leg builds, because coral generators have absurd regen after redlining them (gaining back nearly the whole bar ~3 seconds after exhausting it) and reverse joints can dodge without energy by jumping.