And done again on Detective Legend (hardest mode that you have to unlock by finishing the game once).
Lots of HP sponges and the beginning is brutal, with the tutorial boss being able to one shot your demons (and possibly Raidou but I haven't tried) on a crit.
The Tsuchigumo fight just before you unlock fusion will also filter a lot of people if they're not careful.
Once you get pass the beginnings, it's "just" the normal game but with enemies with a lot of HP that hit harder, Raidou will stay a MAG generator for a long time so I don't recommend leveling strength early on but vitality and magic because most attacks, even from random demons, will kill you in 2-3 hits.
Physical attacks from enemies are VERY damaging here, special mention to Asura, who can remove 90% of your HP on a crit even with 40 VIT on a lvl70 Raidou.
You can still use your favorites to the end but you do need to be careful and abuse the fact that their passives can be changed at will outside of combat and you'll have to micromanage them a lot more than if you used end game demons with a lot of resistances.
Devil Mode from the original is still harder btw
Going to sleep now, I'll end here by saying that the feature that allow you to use several versions of the same demon should be present in more games, maybe with restrictions like in DeSu because having 2 Beel or Shiva can easily break the game.
Also I love Mermaid.
Mermaid!