To add to what the other two commenters said: as the nemeses gained rank in Sauron’s army (from surviving fights with you) the reward you get once you do kill them gets better.

This means you could pick a guy who’s attack patterns you know and are good at countering and start just taking dives when you fight him and once he’s high enough rank you kill him for the improved rewards.

I think this system would work well is a bunch of other game types, but alas we won’t see that until the patent expires.