This Seeking Tech walkthrough will show you how to beat Mutant Fish without getting hit in Sly 3: Honor Among Thieves (2024) on PS5 and PS4.
Use the following video as a reference:
For the first phase, you need to look at the mines he would be sending out. When it stops at a horizontal position, then you need to move to the sides. If it stops at vertical, then you need to move up or down.
In between these, attack the money to deal damage.
When he sucks you in, attack the monkey before you get too close.
For the second phase, you need to avoid the tentacles while dodging the mines at the same time.
For the third phase, attack the rockets first before attacking the monkey. Keep attacking the rockets occasionally so that they won’t fire off and you should be able to beat this boss without taking a hit.