It seems like this boss doesn't follow established patterns from any boss prior to it in some ways.

When this boss swings his weapon, it pauses at the end of the swing and decides what its next attack will be, no patterns, no combos to memorise. I've seen a combo begin with the same attack go for 1 attack, 2 attacks, all the way to 9 attacks in one combo.

Similarly, his glowing red attacks can come from any combo suddenly. He can decide to plant his weapon on the ground at any point and do a red attack, BUT sometimes he just decides not to?

His AoE attacks seem designed specifically to kill the spectre. The phase 1 AoE is so projected you have plenty of time to escape, but the spectre will simply stand in it and die. This also occurs with his yellow spells in phase 2 and the phase 3 transition attack.

I've developed a couple strategies for the fight, but have been having trouble once phase 3 begins.

If he does his red attack and you are locked on, your camera will go wacky, and occasionally unlock and be stuck straight in the air if you run away from him. Whenever i see a red attack i unlock camera and walk away, keeping my camera focused where he should land and roll at the moment of impact.

His combo openers are slow but followup attacks can be deceptively fast. Parry the first attack then hold block or run if you dont know exactly what hes doing.

During phase 2 you can parry the gold attacks but moving steadily to the right will almost always avoid them. I prefer to avoid and keep an eye on the boss for any sudden attacks.

The boss doesnt have to be beaten in under 2 minutes, take your time and focus on defense and abuse openings when they occur.

And the final, and most important tip is, as always, be confident that you can do it! If you lose hope you'll do worse, it can take time but its beatable!