Though it could barely keep up with that sword, it had to deal with the attacks of the other three at the same time, something it could barely achieve at full power, much less in its weakened state.
Be it the swirling mass of red ice that made it feel pain in its very soul, or the white flames that cut through its scales like butter, the huge great sword that seemed to increase in force with each consecutive hit, or the spear that sent out sharp waves of greenish energy, none of the four's attacks were things the monster could handle.
What's more, the party of four also aimed their attacks in ways that even though they missed the Ape, they still mowed down dozens of monsters in its army.
The Cobalt Bone Ape tried running, but the gaping slash wound it received the instant it turned its back showed that running was not exactly an option for it.