A tip: fireballs always jump out of the oil can in Mario's direction. That is, if Mario is to the left of the oil can, they will jump out to the left.
However, if the spring falls completely to the left of the elevator, it should be safe. The attached video should help immensely with this stage. The first bonus prize, Pauline's umbrella, is an easy grab, but the other bonus prizes, Pauline's purse and hat, are.
This will point you in the direction to a cave. (This is almost directly across from Cranky.) At the end of the cave is a switch with Donkey Kongs face. Perform the Simian Slam to open the gate.
The only threat to Mario on this layer are the fireballs, which increase in number as time goes by. From the center, choose the direction that looks safest and climb up to the second conveyor belt layer.
This will knockout all three of the beavers. Collect the crown when you finish. After the fun small brawl, enter the hut behind you. This is Funkys Armory and this is where you can buy weapons for your characters and ammo upgrades.
Mario is determined to get Pauline back, but Donkey Kong hurls barrels and employs fire to hinder the carpenter's efforts. Guide Mario through the gauntlet and up to the top before time and Pauline's patience runs out!
This layer will never change direction. Your last objective in this stage is to climb up the retractable ladder when it is fully extended to Donkey Kong's belt. You only have to reach the belt; you do not have to directly rescue Pauline to advance.
At the start of the level, several fireballs will spawn. Wait for them all to come out before crossing over to the right side of the screen, and there's a good chance that the right side will be safe.
The second hammer should be used often in early levels when there is plenty of bonus time available, and sparingly at higher levels if the path to the far right ladder is too crowded and you need a breather before climbing to the highest girder.
Activate teleport pad number 1 near you and also pad number 2 which is a little bit ahead. The next thing to do is to rescue Diddy of course! But first we need to take a detour.