Or there are a lot of enemies, traps, or tanks. The Horned Reaper is essentially a glass cannon. He can do a lot of damage, yes, but unless there's only a couple of enemies like Thieves/Archers, he's going to get defeated fast if he rushes into combat blindly.
Think of Suria, she's pretty similar in that aspect.
I was thinking of a fight against Wizards or possibly Fairies as support but yeah, in the dungeon, Horned Reapers can't do much. Another weakness for them.