It was a stupid ending to me.
The series would've been SOOOOOO much better if the finale involved a final epic True Last Boss fight against Kriemheld Gretchen with the girls all teaming up, combining forces, and using their power together to vanquish her, probably with them all dying in the process, but the True Last Boss Witch being obliterated and peace being restored afterward. And the battle would be really long and drawn out and cool, and wouldn't be over in like a minute.
Like some sort of long and epic True Last Boss Fight against her, sort of like the True Last Boss of Apokarimon in Digimon Season 1, or the one against Mala-Myotismon in DigimonSeason 2, or the one against Final Dark Oak in Sonic X, or the True Last Battle against Andromeda in Megaman Starforce The Anime, or other awesome True Last Boss fights at the end of a series. But no, it was so disappointing of that wasted potential for something epic and cool like that.