It's the first Altus game I've finished, unless Trauma Center counts, so I started on easy switched it to normal half way through kamoshida because it was too easy. The only reason I struggled was I suck at fusion.
Normal. Felt it was pretty decent difficuty. A few fights were silly easy (Madarame, Kaneshiro and the Sphinx come to mind) but December I actually felt was hard.
Normal first cause i wanted to follow the story without to big a challenge, and currently Merciless where I am trapped in Mementos and can't leave because it is literally the last day to fight the twins.
I started playing on Normal difficulty, but near the end of the game I decided to crank the difficulty up to make the situation more urgent and engaging. I'll continue to increase the difficulty on future playthroughs.