whats a good broadhead sharpener?

What kind of broadhead? I use a Smith's handheld carbide sharpener (like an Accusharp) for my slick trick blades, but some broadheads are more suited to different types of sharpeners.

it does depend on bh design

the bh is a magnus stinger.

All of the blades are removeable...you can definitely get away with something like what I use (accusharp style handheld carbide sharpener). They also make some of the block style sharpeners for 2/4 blade broadheads but I have no experience with them. You could also go to a Lansky system, but too me that's too much effort when the carbide sharpener gets the job done about as well.

would a diamond knife sharpener work?

Heres the one I use pretty cheap and have several laying around.

I bought a Lansky Sharpener for my knives but it also works well for my Slick Trick blades.