This is in Act 5 Level 5. When you deplete the Boss' health bar, she will run to the right and start recharging her health. When she is doing this, you need to run over and start hitting her backpack. If you don't destroy her backpack, she will regain her health and the fight continues. After destroying her backpack, the battle will be over and trophy pops when the weapons/loot/money spawns. Happy Gaming!
Note: You can get another trophy by jumping into the pool.