Act 3 Proper Order

[Ratsplat]

So the general understanding is to finish the story once Act 3 begins, otherwise you'll one shot the final boss if you do all the extra content first. While good advice, I believe you should finish all Relationships until you max them out for they are quick to finish and provide some good backstory before finally seeing the finale.

An optional suggestion is to do all side content in the game once you have the ability to fly, but once you start working your way towards the final boss, I'd suggest removing most if not all pictos and Luminas (depending on how strong you became). Not just to make it more challenging, but because going slow allows you to hear all the voice lines that play out during the fight. One shotting tends to just avoid all that...