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

Solution 1:

Shadow Mitsuo is level 45, so it would be safest to be a few levels above that. Depending on your difficulty (if you're, say, on Normal or Easy) you could probably get away with a little less than 45.

After beating Shadow Mitsuo, you'll be around the halfway point of the game factoring in the True Ending.

There is no real trick to grinding to make it faster that isn't highly situational and out of your control (ie Arcana Chance). (Edit: Though there are some EXP boosting items that you can get from End-Game fishing, though this requires maxing the Hermit Social Link)

If you haven't already, be sure to revisit dungeons you've already cleared to beat the optional bosses for some good exp, a special item and some bonus Courage.

Edit: Also, party EXP is divided between party members, so if there is a specific party member you want to level up, bring them without anyone else and both them and your MC will effectively be getting double EXP. Going with just the MC will also give him quadruple EXP but that's usually very risky outside of NG+.