The human form of Trish could be a disguise too then.Sparda's real form is probably the demonic one though. And the human form some sort of disguise. Or the human form appeared because he sealed his powers. We know basically nothing of him.
We know Trish is a demon. And she has been with Dante since DMC 1. So it's logical that her powers have grown.
And if she wasn't powerful enough to take on a demon for in beginning - she defintly should be able to now.