[EB01-039] - Conquerer of Three Worlds Ragnaraku (Extra Booster: Memorial Collection)

I have 10 DON!! cards on my field while my opponent has 9 DON!! cards on their field, and my opponent has this Character which costs 8 or less due to another effect. Can I activate "EB01-039 Conquerer of Three Worlds Ragnaraku" and use its DON!! −1 to change the number of DON!! cards on my field to 9, then K.O. this Character belonging to my opponent?

Yes, you can.