Earlier in the game, I tried attacking and peeling off two or so Horde Protectors at a time, but this got monotonous fast as I couldn't figure out how many were left. After reaching near the end of the game, I made a frontal assault from the southeastern bridge. Through a series of charms, buffs, battle crystals, and invulnerability draughts, I managed to kill the first large group of fifteen (?) Horde Protectors. However, after killing these, I was still being pestered by one or two newly arrived Sliths. So rather than waste more resources on them, I decided to retreat. Maybe I'll try again after the next to last task.