No matter that Plague Reaper was a Top-1 monster, he was still a Purple Threat monster and not someone who could easily defeat even K3-Phantoms.
Plague Reaper and the Phantoms before him were on the same level and were each other's natural enemies. Golden Light Squad, and especially Adam, were not simple K1-Phantoms, and neither was Plague Reaper just a strong monster.
Plague Reaper couldn't just kill them all in one attack or break their bones with his aura, he wasn't that omnipotent.
Thus, their attacks were dangerous to Plague Reaper, which was why he did his best to repel Erla's shot because he knew what would happen if her attack hit the target.
And... Oskar's massive sword, empowered with light energy from his Second Nexus Trait, was a real threat that could end Plague Reaper's life in one precise swing.
In a few seconds more Oskar's sword would plunge into Plague Reaper's neck, severing his head.
