Issac was coming to his wits ends against the admiral. No matter what he did he could not stop a single attack. It was also apparent that the admiral wasn't even taking the fight seriously. There were so many opportunities that the admiral could have killed Issac, but he just never took the chance.
'Maybe I shouldn't have thrown Luna in the dungeon.' Issac regretted his choice to lock her up in the dungeon as she would have been a great help. His instinct as a friend got the better of him, making him fail to see the actual consequences of her not being around.
"Enough playing." The admiral suddenly spoke his first words which only meant bad news.
In a panic, Issac entered his transformed state to give him the extra edge. He didn't want to use it, but now he had no choice.