What Quinn was thinking of doing was summoning his demon from, but there were a few reasons why he was against doing this. For one, Mundus was watching, if the form was powerful as he thought it was, he feared that the celestial might go back on his deal and consider him too much of a threat.
In a way, it was true as well considering Quinn wasn't in control when using this power. The only reason why he was fine with using it now, was because none of his family, friends, or even other life forms were around apart from the phoenix.
The second reason was because it was a wild card. It wasn't a guaranteed win and could possibly do nothing against the phoenix.
'Do I really want my final moments to be this… a memory that I can't even remember?' Quinn thought as he hesitated. 'But I shouldn't be thinking like that, I should be thinking with the mindset that I'll win, that I will return to my family!'