In a straight fight, DG's weapons would give him the advantage. However, Hellboy's regeneration and mystical powers could help him endure and counterattack. The winner depends on what kind of fight would play out.
If Doomguy just straight up charged at him, Hellboy would dodge his attacks and use his powers to back-shot him and then finish him off. If Doomguy used more a more strategic and defensive attack style, then maybe he would win.
Imo Hellboy wins mid diff, Slayer's all offense and barely any defense, while Hellboy is balanced very evenly in those aspects, giving him the edge, and not to mention Hellboy murdered every being in Hell once