You must wear "Hands of God" weapons in your right AND left hand for the 2-3 boss fight. If you only wear one glove, this trophy will not pop. To get the weapons, you can most easily find them in a cave with large dragon skulls just before fighting the Flamelurker boss, on a corpse guarded by three Rock Worms.
Shown in this video N7 at 40:40: Once you have the weapons, you will fight the Dragon God just after defeating Flamelurker. The dragon is a rather lame puzzle fight where you must fire off two large harpoons into the boss, to which the dragon will eventually screech and lay his head perfectly in place for you to melee him in the teeth. Now you just equip the boxing gloves and punch him 2-3 times then back off quickly. This process is tedious as well because his breath is still hot enough to hurt you (a lot). Back off and heal if you get burned, then just come back and punch him till he's dead.