Castellum on the Leviathan. Shoot the crit spot on a phalanx shield 4 times to proc perfect fifth. Then shoot a red bar once in the leg. They'll die from the burn, shoot the shield 4 more times to proc it again, another red bar kneecap. Rinse, repeat. Easiest method. The shield allows you to easily proc the 5th repeatedly and the red bars respawn endlessly.