The little baby is Dumbo I believe
The only strategy I know:
Stay above him every time you're not attacking him.
Calm down.
Make things bright when it gets dark.
Kill the little squid.
When things are calm jump in front of his mouth.
Then immediately jump a bit back so that you won't get grabbed by the tentacle.
Shoot at the... thing in his mouth.
Keep repeating the jump and shooting until he spews ink.
Go back up again, cling to the wall, change to sun form again and lighten the place again.
Then repeat the stuff again.
When you get hit by the little squid, go back up above the octopus and kill the thing, and lighten the place if the place gets inked by the octopus.
So, tk'dr, stay above the octopus, keep the place lightened, kill the small octopus each time, jump in front of the mouth/beak of the octopus, jump back immediately and shoot, then repeat.