It isn't that different from what you did to get the Tortolodon at the start.
You prep as usual, with no delay on the encounter tool's timer, and you need to time the start of the battle to that timer.
The added difficulty just comes from having to keep the cursor on the fish while waiting for the timer to run out.
So yeah, if you want to see how it's done, Hap.dev did so in his video at 43:58 (the comment above yours)