Other places to use Octo besides Newties?

Any boss that weapon def's, except Pianus. You can't really do anything anyways while it's up, so might as well place an octo. But you also run the risk of it attacking spawn, if it applies. Especially at Pianus, it can mean an instant kill. So make sure you know what the boss can do before summoning.

lol pineapple Pianus. I killed my party by placing an Octo once. I have a habit of putting it down whenever I can f1

I did it again when we boss PQed but luckily I Recoiled to the edge of the map so my Octo stopped shooting.