I’d say Abel would win that fight by far. He can command all demons, is a very good tactician, as well as the power to summon Beldr, a demon that is immortal. The only thing that can kill it is in Abel’s hands. Not to mention Abel has a lot of allies like Naoya and Atsuro, people who are incredibly intelligent as well as capable demon tamers. This coupled with the fact that Abel succeeded in killing god, is why Abel would destroy the Demi-Fiend.
Abel doesn't command all demons. There are some that opposed him even when he did attain the throne. Also having an immortal demon doesn't mean anything. Demifiend killed Kagutsuchi, who is also immortal. There has to be something more impressive that Beldr can do aside from immortality. Abel may have friends, but you addressed it like that's what all the fight is about, it isn't. One of the rounds have them by themselves. Take out his friends and now what happens? Also where is it stated that he succeeded in killing god? Nowhere does it say this kind of thing. There might be some implications that he won, but that's kind of a moot point since Demifiend also has implied to have defeated god.