Subaru thought he was in some kind of RPG. Though I never played one, some anime told me bosses have two sets of paterns and become far troublesome when in the second phase (was already working that way in shmups I played almost 30 years ago...)
So I was waiting for that second phase and was sure that attempt to get rid of the whale would fail as they were not able to finish it, eventhough it's not that fast and not that active in attacking power.
But the huge shout clearly felt like a call... equivalent to the one the whales we know irl use.

The reset Subaru used seems to have changed the save point right mid battle. So it's now impossible to prepare for multiple whales, or even persuade people there's more than one.

Feels like a timeline with no way out, even with reset by death...
Because I do not see what can be changed that will give Subaru and the others a way to win. They need a way to kill the whale quickly enough so that there's no call to others.
And no one there felt strong enough for that feat.