Tip: If you equip the Eagle Eye Exospine, you can purchase extra bullets for your clip, but when you unequip the exospine your extra bullets will still be there.

I did before each boss battle. I unequipped the exospine for something else that I wanted to use in battle like Judgment-Type. Unloading a full clip of explosive shells on the boss will fully deplete their shield. Then I use Tachy mode, burst, beta, melee. Then when they regenerate their shield, I unloaded a full clip of stingers which fully depletes their shield again. The ranged weapons are really effective against bosses.