This is story related and can't be missed.
*** Spoiler - click to reveal ***After conquering the fort at Gorgoroth and beating the Rogue Nazgûl sisters, you'll watch a cut-scene in which Talion asks you to take him back to Gondor. Another cut-scene shows the two of you arriving whereupon Talion refuses to give up his ring and you begin to fight him.
This fight is a bit annoying as Talion constantly throws his little daggers and some bomb things at you, making it hard to heal yourself. He also dominates nearby orcs and raises any orcs you kill. Once you've lowered his health a bit, other Nazgûl join the party and start fighting you as well.
Some tips for this fight:
Use your Light Blast ability (hold and release ) to infuse nearby orcs with light and then quickly kill them with Light Bolts (tap to throw them). This should help raise your Elven Rage meter up.
Once your Elven Rage meter (the yellow bar) is full, activate it ( + ) and go to town on Talion with standard attacks. While this is going, anytime your Might meter fills up (the little orangish bar over your health) use an execute move ( + ) on Talion to lower his health.
The Cone of Light ability (hold ) works well against Talion so long as there's no one around to smack you while you're aiming it at him.
Don't bother fighting the other Nazgûl. The object is to lower Talion's health so focusing on anyone but him (and orcs to build your Might and Elven Rage) is a waste of time.
If you find yourself low on health, run around a corner and use your Radiance (hold ). If this gets interrupted it won't work, so if you need to you can run really far away. At one point I ran so far (in the direction of the main fort, I think) that the game warned me I was leaving the mission area. I healed up and ran back with no problems - but I wouldn't advise lingering with that warning on-screen very long.
Once you defeat Talion you'll get a final cut-scene, after which this achievement will pop.
Overall it's not the hardest fight in the game, but it can be a tad annoying. Hopefully these tips help if you find yourself struggling. Good luck!