According to Koops's 302nd post, made on August 05, 2017, at 03:27:31 PM, Waterblight Ganon took him days of preparation before, but he found out he wasn't making flurry strikes and not using his shield correctly. Plus, due to his overusing the best bows he could find, the strongest bow he had in fight was the traveler's bow and so it took him forever to kill him. Fortunately he had already aquired an Edge of Duality by this time so the first phase wasn't that hard for him.
Nonetheless, Thunderblight Ganon was way more annoying to fight, especially in the last phase, where Koops had to time almost perfectly or he was very likely to almost die.
The post was made in a hidden childboard called "Ask Board", hidden from unregistered users, and was in a thread called "Ask not what Luigi 64DD can do for you, ask what you can do for Luigi 64DD!"