Bosses can be defeated by dealing damage until they die. However, if you kill many of these bosses in a certain way they will drop unique items that cannot be otherwise obtained. These items can then be used to either craft a weapon or a mod. Check out our Alternative Boss Kill Guide to find how to perform the alternate kill to get these hidden item drops.
Ent:
Destroy Ent's legs before it dies. Focus on the legs, not feet. Any damage works.
Singe:
Destroy Singe's tail before it dies. Any damage works.
Brabus:
Kill your friend when prompted to get a trait.
Brabus:
Trade a Pocket Watch before the fight.
Maul (The Pack):
Shoot the NPC caster in the head, grab the module.
Claviger:
Defeat the boss before it absorbs any of the other enemies.
Harrow:
Stagger him by shooting his legs. When you do this 3-4 times he will crouch on the ground. When its crouched, run to its back and interact to get the spear.
Undying King:
Refuse to give the king the heart or give it to the queen.
Ixillus:
Defeat both bosses at the same time, before one enrages solo
Totem Father:
Totem before the boss area. Blue rewards with gun, red with spear
The Ravager:
Make the boss fall asleep by playing the lullaby.
Check out our Crafting Guide to find out everythng you need to know about crafting in Remnant: From the Ashes.