Roughly what levels should my party be at for fighting Shadow Mitsuo?

I've already looked up some strategies for the boss, but I've been beaten several times without managing to lower the "main" form's HP very much. I can't seem to squeeze in enough damage to stop the boss from rebuilding his "Hero" form, and then Gigadyne eventually gets the better of me.

For the first attempt I used the original four Investigation Team members, and then I swapped out Yosuke for Teddie to make healing HP and Fear/Exhaustion easier, which helped.

The party's levels are 30-37. I'm guessing there's a lot of grinding ahead of me; which members should I focus on leveling up, and what levels should I shoot for? Any advice to speed up grinding and/or lessen potential for frustration is welcome, too.

While I'm asking, I'd also like to know how far into the story I am, roughly. This is my first ever run through the game, and although I've been playing casually I've taken three years to get this far. Without spoiling much of what's ahead (I'm aware that there are multiple endings but don't know what the criteria to get them are), I'd like an estimate of how many more playing hours are left to reach a good ending.