I've been doing some trial and error after Smithy, looking for the hidden chests I've found a few bosses. Just wondering if anybody wanted to compile a list, perhaps by difficulty? Feels like I missed a few available before smithy too, Like Bowser's solo fight. any help would be great. Here's my small contribution 
2 Black Shy Guys (Kero Sewers and Roseway) Shiva (Ice Cave, special blue pipe), the Ice Tonberrys (Ice Cave, a different special blue pipe), and the Moleville Tonberrys (Punchinello room, before the mine cart ride as well as dropping down). Also Master (Mushroom Kingdom), Jinx (Monstro Town), Croco (Nimbus Land), and Valentina (Booster's Tower) (solo) These are what I've beaten, now lvl 97-100. I found Brachosaurs and the mysterious girl in the Forest Maze too, but they were too strong to be fun yet.
