Beat the game. This unlocks in 7-3. It's a relatively easy boss fight (I found a number of others more challenging). The big trick with him is that he'll require you to use every skill you've collected in the game. He has four unique phases but none of them are that hard.
Here's a video of the fight. the first 10 minutes are the fight. The rest is the ending. Be aware the video has spoilers for those concerned with the story.
All credit on the video belongs to Sooo Mungry