There are multiple events were you can show Romanos how far you have come to break his will, once you have he will wind up accepting the corruption through the pain of being forcefully corrupted. There is one scene at the town by the tower (I can't recall what the name is) once it is fully submitted to you (there is a glowing spot to inspect by the center building), and you can go to his cell after meeting the Demon Queen where Misaki will give him a dressing down of how pathetic he is compared to the MC, and another can be seen if you fully corrupt Rina and Yukako at the same time, also there is a scene with each of them where he runs his mouth to those the respective girls are trying to support (for no one's sake but his own, even trying to throw the rebels to their death for a fruitless attempt at another go at the MC, if not for the girls admonishing him for such actions) when let out of the castle for a moment where they each dress him down for his character before returning to the castle for each of their corruption events with him around (one scene per character). With his will thoroughly broken from these events, he has no will to properly resist the corruption as his ego breaks to the corruption.
TLDR; Have various interactions with Romanos to steadily break down his will so that all that is left to break is his useless sense of self through forced corruption.
