Take into consideration their willingness to participate in the festival, how easily they get moonscorched, how willing they are to team up and consider their skills both how they work in-game and how they would in real life. Left to their own devices in the game without a player character, who would win and why?

Considering that August seems the least inclined to participate, my answer would be Marina, with O'saa as a close second. O'saa knows about the festival and is strong, but he only reluctantly joins you when he's near death. Marina not only knows of and believes in the occult, giving her access to the ritual circles, but she is one of the few who can be recruited day 1, and would most likely party up with Levi, who she can later easily kill if necessary, moonscorched or not. She also doesn't moonscorch easy.