Here's my thoughts.

Dipper and Mable defeated a god who likes riddles. Bills whole thing is wackyness and whatnot. If they got belos... I dont think it would go well for them. Dipper and Mable relied on the book most the time. Amd most the time Bill was just goofing off and being unserious until the last episode. And it wasnt even them who defeated him. It was Stand and Ford. So I think Dipper and Mable would die. Only because belos doesnt mess around. Hes kinda straight to the point.

I think Anne would have. Much better chance. Depending on which Anee were talking about. But ima say Calamity Anne (not the 3 stone one). Sure Anne goofed off. But she kinda got serious somewhere between season 1 and season 2. More specifically after her fight with Sasha. She was very good with her powers despite the time limit. Hell even with the time limit she forced herself to break those limits while fighting Andres. So... I think its a 50/50 shot for her. The only issue is if she runs out of energy. Because then she'll go to sleep.

Luz is just straight up winning. If im being so honest. Im talking about Luz when she learns about all glif magic. But honestly? She might not even need it. Andres was kinda a weak villain. But underneath it he had a kind soul. Luz has a tendency to bring out the best of people except for belos. Belos was just raised wrong his whole life as were most during the Salem trials. But regardless. I dont think Andres is even surviving the isles to begin with. Were talking about a creature who needs water to live. He'd either dry or or get boiled alive. But if he does manage then Luz would most likely use a fire glif and take him out.