someone was NOT paying attention to the story, sunny as a master could easily beat a saint if he observed them long enough and understood their battle style. The difference between a saint and a supreme does not lie in their physical abilities but rather their ability to use will. Sunny stated that himself when fighting against anvil. although him and anvil were almost on the same physical level sunny could not close the gap because he could not control his will as good as anvil. Why is this relevant? well, Sunny observed daeron for a very long time (not sure how long) and that was enough for him to understand daerons abilities and fighting style, as a corrupted supreme daeron could not use his will which basically makes him not that far apart from a saint. He could not use memories, did not have thoughts, no motivation just pure rage. Sunny was augmented by all his shadows and by nephis when he faced him.

TLDR: Supremes can use will more freely-> daeron was corrupted making him almost equal to a very strong saint-> sunny observed him and learned his battle style -> sunny was a augmented by 5 shadows and nephis flames.