Weak huh? Well, I'm not very good at servant balancing truth be told, so I usually go with more conservative load outs. If you've got a specific idea of which stats need buffed and by how much I'd be happy to hear them. The horn itself isn't really the Noble Phantasm, but the idea behind it is. The concept that upon blowing this horn reinforcements will come. It's a symbol of hope as the name says. Being a one use buff, it fits that Roland refuses to blow it until the circumstances are incredibly dire. 

And I did not know Durendal had official stats, I much prefer my version but I don't know. Can I keep mine, or do I use that?