NOTE: This walkthrough only entails how to defeat the "real" Robot Master, and get the Beat Letter.
This is an unusual stage because, early on, there are two separate paths you can take. Neither one really has any added benefit over the other, it's just a matter of taste really.
The normal path (simply climb the ladder) is the obvious one, and you'll encounter the usual assortment of baddies along the road. All in all, it's longer than the hidden path, but probably a little more simplistic than the hidden one.
To get the hidden path, use Rush Power to break the block in the right wall below the ladder. You'll enter a hidden path. This is a shorter path than the "normal" one, but you'll have to battle a frog-like mini-boss. Still, he's not too tough to bring down, and combined with the fact that this road seems to be shorter, it should be the ideal choice.
Eventually in the level, the paths will split again. Use the Rush Jet to get to the higher area, and use it to more easily navigate the challenging series of conveyors and bumpers that await you. The Robot Master is waiting at the end of this high road.
Yamato Man can be tough, but if you've got the Silver Tomahawk, he's pretty easy to dispose of. He has two main attacks - jumping and firing spear bits, and flinging the tip of the spear he holds - which he will always have to run over and retrieve. Get in close and unload on him with the tomahawks, while dodging his attacks. He'll bite the dust pretty fast.