She has more mana and could probably take him in a fight since she doesn't use complex spells, she literally just infuses her punches with magic just like how Charmy did in her fight with Lira.
The drawback to Yami's magic is that it's really slow especially when compared to Mereleona or Vetto, but if he has a chance to get off his Dimension Slash it's pretty much an instant finisher. It'd be one Hell of a fight though his nickname is The God of Destruction after all.
Speed wasn't a deciding factor in Yami vs Patri and he is way faster than Vetto so saying Yami is too slow is an invalid argument and a complete lie. Besides Yami would wipe his a## with Vetto at this point. Nobody Mereleona has defeated is even in the same league as Yami by themselves. Yami would slaughter Mereleona like any of the other Captains one on one.
Asta's demon isn't "considered a lowly demon." He mocked the kotodama demon by calling him chief. The kotodama (word soul magic) demon didn't even recognize the demon, which adds to how mysterious Asta's is.