Hmm, well here's my party for reference. I didn't grind much near the end (I don't really have that much tolerance for heavy grinding), but earlier in the game (after taking over the right side of the map) I spent a lot of turns just stocking up on Ouki so I could buy out the shop.
At Turn 127, when I had Makai down to their last territory, I had Yukari at level 33, 17 units from levels 24-31, 3 units from levels 21-23, and 4 stragglers from levels 14-17 with a total power level of 25097.
VS
Makai Army with named units from Levels 33-35 and a power rating of 20909.
I think I beat them relatively easily at these levels but then I ran into a wall at the final final boss so I took some necessary turns to grind.
At Turn 142, right before I took them down to 0, I had Yukari at level 37 and 6 units at levels 32-35 (Yuyuko, Eiki, Aya, Suika, Reimu, Marisa), with the rest of the units unchanged. Total power level of 26196.
Meanwhile, the Makai Army powerleveled, with named units at 37-38 () and a total power level of 23175.
It was a little harder to take them down at those levels, but I did it eventually. And the final boss still caused me trouble. Suika and the rest of my main team died almost instantly after taking down 2 helpers and about 500 hp (which was instantly healed!), Koakuma not soon after along with my Pressure generic, and eventually the only reason I won was because Yukari consistently hit for about 20-100 damage every turn (while activating Yakumo Frays to counter the boss's buffs), while the boss (who must have healed over 1k total) kept wailing on a Level 17 Meiling who healed herself every turn while another generic kept pumping Rally and I suicided all my other units on attack (yay for Flandre). A 30 minute plus battle for me, ugh.