When it comes to Powers and Abilities , Maou had a great power than Emi .
As the Demon Lord, Maou possessed great demonic power. Demonic magic is extremely strong, and each user seems to have a unique coloured aura to indicate its presence. For him, that color is black.
On Earth, he lost most of his powers and was weakened to the point that he could only function as an average human. Finding magical sources was near impossible as magic is largely only present in the form of fiction. However, his powers can be temporarily restored by feeding off negative emotions like fear and despair from sentient beings in the surroundings, such as during the battles with Lucifer and Sariel.
Maou is able to cast so many high-class offensive and defensive magics to attack enemies or to defend himself from harm with ease.
Maou is surprisingly intelligent, able to piece events together and form a logical conclusion with relative ease, such as when he figured out Lucifer's motivations for the mugging incidents around the neighbourhood to accumulate power.
While Emi's human body is more fragile on Earth than it is in Ente Isla ,shown to be capable of flight, but its limitations (such as whether it uses up her Sacred Power or not) are not clear as of yet.
to be exact Emi can't defeat Maou.