It would interesting to see the orignal Devil May Cry and Puella Magi Madoka Magica do a crossover, however, witches are formed from human females that became magical girls, so the real enemy would be Kyubey since he and the other Incubators created the magical girls and cursed them to live in despair. To finally end the magical girl's suffering just stop them at the source by killing Kyubey and the Incubators which wouldn't be a problem for the youngest son of Sparda.