Cere Junda VS Grand Inquisitor

Cere as of Fallen Order(duh)

and Grand Inquisitor as of his final appearance in Rebels.

The battle takes place in an open field.

GI should win, honestly. He's a better duelist and he's powerful enough that Cere can't just stomp him unless she's fully tapping into the dark side.

Yeah GI

Cere if she taps into the darkside. Her bringing Vader down even for just a little bit is far more impressive than anything the GI has done.
Otherwise GI probably wins.

GI should have this unless Cere somehow becomes amped with dark side and unleashes fury. Most cases it shouldn't happen so GI should take this.

Edit: It might be possible the GI knows about her connection to Trilla so if he taunts her it could backfire hysterically.

almost Ventress level wins

Cere has some pretty impressive Force showings. Slowing Vader and briefly driving him to his knees when tapping into the dark side, and creating a protective barrier that nearly halted Vader's lightsaber while drawing from the light side. Granted, those were both from very intense moments where her performance level was likely well above her base, but it still speaks to the depth of power she has at her disposal. 

The Grand Inquisitor is more consistent, and he's almost certainly a better duelist. I'd favor him to more frequently come out on top, but any match up where the Grand Inquisitor strikes the wrong nerve, he gets destroyed by her superior raw power.