The bosses in Inkwell Isle 3 are as follows:
Rumor Honeybottoms (Honeycomb Herald)
Captain Brineybeard (Shootin n' Lootin)
Sally Stageplay (Dramatic Fanatic)
Werner Werman (Murine Corps)
Dr. Kahl's Robot (Junkyard Jive!)
Cala Maria (High Sea Hi-Jinx!)
Phantom Express (Railroad Wrath)
Beating the Run n' Guns aren't necessary for the this achievement, but you might as well go for them to get some coins to buy new weapons and some achievements later on.
Here's a guide for finding and defeating all bosses, the guide is also a A+ Walkthrough, so it might be useful for other achievements: