Story related and cannot be missed.
This boss fight is at the end of level 5 and it might pose a bit of a challenge but not much. You are on a roadrunner and you are tasked with dodging debris, bombs, mini gun bullets, and enemy minion fire while you are shooting at the stage coach that Bad Bill is riding on. To dodge the debris, it may be easier to jump over some of it. For the mini gun fire, just run from side to side to dodge it. Killing the enemy minions quickly is vital because they drop items that replenish your health. All in all it's not that hard to defeat him.