Gotbuck, with a barrel that long, you could beat them to death at a safe distance... what caliber is that rifle?

Personally for me it would be my White Ultra Mag .504 topped with a Nikon Omega Scope. I would load a 500 gain Bull Shop conical, 120 grains of Triple Seven 2f, with a wad, and aim for the top of the heart. That big conical should penetrate and do the deed.

Also I would make sure my guide had a good modern rifle and was an excellent shot. And clean underwear in the land rover in case things got hairy...