If you want to train fists, the best way is to make the Fenris set and flesh rippers with it. You could also punch some boars and whatnot in early game, but that's not really productive, just funny. The Fenris stuff is available from mountains onwards, but are almost viable all the way through mistlands. The speed bonus also makes it great for running around in all sorts of previous biomes regardless of what your objectives are, allowing you to benefit from the skill increase to actually deal damage with the ripper-fists.