This stage is Lava Rock Lair (4-Castle). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
You will get to a switch, stun a rock monster and place it on one of the P switch. Land on another and you will lower a bridge leading to a green star.
Green Star 2
Jump into the mystery purple box after the first pipe. Stun the rock monsters and place them on the P switch while standing on one yourself.
Green Star 3
You will see the last star to the left of a long bridge. You will have to stun and pick up one of the rock monster, toss it to the green star to collect it.
After taking the long horizontal pipe, head left and climb up the wall as Cat Mario. You will have to climb up ledge higher to grab the stamp.
Boss Fighting Tips
The key to beating the boss is to be patient. Toss the rock monsters into the boss to deal damage.