Gameplays - Lies of P - Puppet King Romeo Boss Fight NG+ PS5Pro

Tactics:
Come in with some heavy throwables and anti-shock equipment. If you have stagger-increase grinding stone that's also going to be handy.
For phase 1 - if you go alone - it's going to be rather tough because of a large health pool of the king. Still, if you've beaten him in normal game, you'll do so here albeit with a more stretched skirmish. Avoid wide swings and unblockables, throw some balls or pipes at him as he's about to be staggered and it'll be fine.
Phase 2 - highly recommended to have specter to aid you, so pick a healing cube and keep him alive through phase 1.
During the fight, if Romeo focuses on you strife left and use hit and run tactics. When he's going for the specter hit him from behind to stagger him. And that's basically it.
Puppet string will work wonders if you use it at a proper time.