Stuck on the 'Crimson Tyrant' boss in Elden Ring – Am I missing a key mechanic?

I've tried everything with my Faith build – incantations, summons, even respeccing to try a strength approach, but this boss just walls me. His second phase is brutal, and I feel like there's a specific pattern or vulnerability I'm completely overlooking. Any advice on breaking his posture or exploiting a weakness would be a lifesaver!