I liked that the boss had phases, and that the attacks were differentiated enough that you have to keep on your toes the whole time.
I wasn't a big fan of the JKL controls not being in order of size -- i.e. I'd have liked them to be JKL from smallest to biggest, or vice versa, since that's easier to intuit in the middle of combat, as opposed to trying to remember which is which.
Though, speaking of the scaling, I'm not sure the middle size is actually that useful; the small size is good for dodging, and the large size is (presumably) good for damage, but the middle size does less damage and can't dodge the attacks, so it doesn't seem terribly helpful. Also, when you hold space, the octopus shoots for a little bit, and then stops, which was kind of weird/annoying.
Great work for a first entry to GMTK Jam ever!