I fail to believe Dante was unaware of the Devil Bringer and caught off guard.
In my eyes and with reasoning I think Dante new full well that Nero possessed the Devil Bringer and wanted to see what Nero was capable of, thus not taking the fight to him as it were. We see swords clashing but never any real intent on the part of Dante, he willingly takes punches to the face and yet refrains from kicking Nero's behind, we've seen to which extent Nero beats Dante up, yet he doesn't truly fight Nero. We have all seen him fight, we all know up until this point that Dante shows no mercy to his enemies, even to his half demon Twin, even though he loves him. So why didn't Dante fight Nero seriously, could it be because of the Devil Bringer and the Possessor or merely because he knows the story, he knows what the Order are about and wishes to enlighten Nero, with no intent to kill him.
What I have taken from that trailer is simple, Nero is using all his might at that level of the Devil Bringer against Dante and yet Dante doesn't fight with any conviction, taking the punches thrown at his face, taking all of Nero's hits and in the end, happily leaving with a smile and humour in his voice.
Question is, WHY?