^by what logic?
Dante's firepower doesn't do more or less than the same weapon in Lady's hands. Unless Dante applies demonic power via. Charging, the shots are the same.
Lady is lithe and agile, because she is human she understands the value of not getting hit, whereas Dante is happy to take a hit if it results in a Killing Blow etc (or blinding blow...see Beowolf).
I'm not saying Lady could go toe-to-toe with the likes of Arkham, Beowolf or Nevan...but despite the size of them Gigapede, Heart of Leviathan and even Geryon to some extent are lesser demons in comparison to the former soldiers.
Lady is small enough to nimbly dodge Gigapedges blasts and retaliate in kind.
Heart of Leviathan is a long shot but she could easily leave a few Grenades w/o their pins to give that beast a massive heart attack...and she would'nt lose DT in the process rendering on of the Glands useless.
Geryon is a toughie, but again, she is lithe enough to use Kailena-Ann to deal some nasty Damage to the Horse and Carrage...despite the rocket retailation.
Sadly, this is a debate that cannot be resolved because alike so many it is all down to speculation and opinons of the characters abilities, there are no facts to suggest otherwise until DMC5 atleast ^_^