at the start the only thing he could do is mimick other nightmare abilities. This resulted in his body rejecting the source of that power, his hand which served as a connection to god. Basically he was using someone else's power.
Recently he has however been using unique powers similar to those of other nightmares, like when he forcibly merged with John. These result in serious feedback to his whole being and it was even mentioned that it might turn him into a nightmare. Hence the comment that he'd even abandon being human for Merry.
These are the only real abilities he has, he has been using god's power most of the time, and only recently changed the nature of it and making it his own, which presumably cost him being human. God just said that he did expect him to get strong and put up a fight, knowing full well that yumeji can't possible beat him through the power he got from him, he'd just be an inferior copy. So his unique powers would most likely be required to beat him.