How do I add Fang to custom levels

[Iced Time]

So I've been trying to make a mod called "Fang-o-Fier" It's like replacing all bosses with fang. Though black core 2 is the only level I can add fang. So how do I add fang into other levels other than Black Core 2? (Context: when I add Fang to other levels that aren't smth like black core 2 Fang just dies before you can even fight him)