This boss will adjust which attacks it uses based on how often you get hit by them.
I'd still use an arena for campfires, heart lamps, and stars in a bottle.
But now that you've beaten it, you likely won't need any of those, the next boss is rather easy, but the 2 after it... Good ing luck.
Thanks, I'll need it! Just took down Purity again in what felt like a more... "standard" run (where it spams a shield every few seconds so the last 20% of the fight is 80% of the length). Still don't understand why they didn't happen my first takedown, but I think I've finally gotten used to avoiding most of its stuff.