Try dog nail clippers... the ones you insert the nail into a hole... that way the beak should cut off straight... OR a dremel tool with the sanding drum.....
Is that bird a pheasant ?? Is it cooped 24-7 ?? have you tried one of those white stones, that attach inside a parakeets cage, for them to sharpen their beaks on.... don't know it they work for game birds...
Now you may have noticed, I don't know squat... a newbie here... just trying to think of stuff I've seen over the last 6 decades that may be of help...
Dave