And Sailor Cosmos in her current life is a coward. However, unlike sorrow, fear can be lethal in the hands of Sextus.
Against an unknown opponent, Cosmos would already have some fear (otherwise her powers would be weakened by underestimating an opponent). Amplified by a master furycrafter, she is out of her depth. And even if she's able to prevent herself from dying or running, Sextus can use his other skills. Such as using aircrafting to suffocate her (or become invisible to her), he could use earth or wood crafting to trap her while she's distracted by the mental battle. Or even watercrafting to change his appearance to augment the fear. In terms of power, she is weaker than Sextus. But she's facing a genius whose greatest strength is his ability to manipulate his opponents into defeating themselves.
