One of the benefits of the Polearm Master feat (PHB, p. 168) is:
While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.
One of the benefits of the Sentinel feat (PHB, p. 169-170) is:
When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn.
So if an enemy aproaches, you hit it with the opportunity attack from Polearm Master, and because of Sentinel it is now stopped 10 feet away from you. They can not hit you, you hit them on your turn and move back 5 feet.
Repeat at will.
Is this unbeatable in a duel? Am I not seeing something?
One way to mitigate could be to move to 15′, then Drop to Prone and crawl to 10′ provoking an OA with Disadvantage (you are more than 5′ away). And then stand and move to 5′. Attack.
Orcs are especially good at this, due to Aggressive.
Obvious downside is that you may end up Prone with 0′ speed left. OTOH if you are, they need to close to 5′ to get rid of their Disadvantage. At which point you’re good to go.