Work your way to level 5-3. (It would be a good idea to choose Luigi for this level because of his superb jumping skills.) Go up the ladder at the beginning of the level, and then jump up onto the ledge above the ladder. As soon as you get on top of the ledge, grab the first grass on the right. It is a potion. Use the potion and go through the door. Now, while in sub-space, go down through the jar and voila! Now you're in Level 7.