Draw Slasher

Story related, cannot be missed.

Octo is the boss at the end of Chapter 3. His attacks involve tentacles rising up from the ground. They are charged with electricity, so touching them or attacking them will deal damage to your character. You need to avoid the tentacles by moving side to side until each attack pattern is done, after which the tentacle will momentarily go limp and you can slice it off. This will cause damage to Octo, and his next tentacle will attack, with a new pattern. Once you have sliced off all his tentacles, the battle is over.

Completing Chapter 3 will also unlock the next 3 Challenges (6-8).