Old West Boss Fight: Bullseye
After completing all non-race tasks around the West (except Speedy Specter, which counts as a regular side quest), follow the Boss marker to confront Old West Bullseye.
Bullseye has a lot of silver-armored goons at his beck and call, so anti-silver bombs are a must- they're also good for dazing opponents on the initial throw.
Bullseye himself is a cagey foe- stealthing when he takes too much damage, and deflecting most distance attacks. Best bet is to get in close for some melee damage whenever he's visible.
Keep slicing away at him, while clearing goons to keep them from firing into your spine, and you'll eventually put this no-good down, unlocking him for purchase and getting a Who's The Boss point.