You can break his arm by failing the first melee duel and again and again until his arm is broken and then you can now proceed to beating him in melee duel....you have also the advantagae to gain more weapon experience

One technique i use to break boss parts is i use Agni ultimate spell to stun bosses for a couple of seconds then i attack the part with arts...if you dont have Agni summon then just be patient with attacking the part you wish to break...

It's much easier to destroy his arm with a 2H sword and poleaxe but you can also break it using SnS. Target his arm and spam on the first BA (where your character turns twice).

I know it's hard, he usually died before his right arm break.

Still, I suggest you use the 2H sword, and keep targeting his. Do melee duel if you want, just keep target his arm and hit it.

You said you lock on to his right arm all the time but are you certain you're hitting it? Locking on to a certain monster part doesn't mean that every swing hits that area...Try to position yourself on his right hand side for better accuracy..And like the guys above said, don't finish him off until you break his right arm..good luck!!!