As you explored Bob-Omb Battlefield, you likely saw some metal bars by the Chain Chomp and if you looked closer, there was a star behind it. Getting behind those bars cannot be done, unless you use a glitch, which is not exactly easy to do. The legit way to get the star, though, is by using the Chain Chomp itself. However, if you stand in front of the metal bars itself, you see that the Chain Chomp cannot reach you normally. The goal for this star is to set the Chain Chomp free, which is a little trickier than it seems, although it is quite simple.
The Chain Chomp is attached to the wooden pole and the only thing you have to do is perform a Ground Pound on it three times. The only problem is that the Chain Chomp won’t be too happy that you’re in his territory, so it will be trying to take a bite out of Mario in the process. Another problem is that since you’re in a bit of a hurry, you will sometimes not get a good read on Mario’s position over the wooden pole, meaning you might miss it a few times. Once you pound the wooden pole into the ground, you see the Chain Chomp go free, but only after busting the iron bars, giving you access to the Power star.