The hordes of hell? Then I'd say it would be safe to assume that these demons would be unphazed by bullets (no nervous system to rupture to life to bleed out), and heat and explosions would probably not be of much use either.
Assuming by horse of hell you don't mean a zombie invasion, then one would need a weapon thats sole purpose is to either destroy evil or capture souls. So in that case I'd have to go with the sword of evil's bane, the Master Sword. Or something from some game that absorbs souls, theres been quite a few of those.
To be fair that book from Avalon Code would work as well. Smack one thing and then write in the book that it is made of like, Gummi Bears and boom no more threat.