This is the second blockade level in the game, with this one pitting you against three giant Galoombas. Despite their large size, they are no different than ordinary Galoombas, meaning you will need to attack them once to flip them over, onto their back. If you wait too long, they will eventually return to normal, so move fast.
Once the Galoombas are on their back, simply running into them will defeat them. Note that you don’t need to actually jump on them to make them flip over, as any power-up will do the same thing, be it a Super Bell or Fire Flower. In fact, even rolling will work, so you don’t actually need a power-up at all for them. Despite being surrounded by lava (don’t fall in), you can’t knock the Galoombas into it. Defeat all three of them and the Green Star will be yours.