I've beaten Wario Land 4 a few times [Wario Land 3 is better].
To beat the Golden Diva, use this following strategy:
When you enter the Golden Diva's lair, she will appear in a cloud of mist. You will then have 6 minutes to beat her.
The Diva will have her face covered by a large fan. 4 heads will be rotating around her face. Each of the heads will bounce around the screen, one at a time. Attack the heads [by pressing B button] to knock it at the Golden Diva. Repeat this until the fan disappears, then her true face will be revealed.
During this time, she will try to attack you with a variety of different objects. The first object she will throw at you will be a small green flying creature. Knock the creature into the air and it will hit the Diva in the face, knocking away a few of her health energy.
Her next attack will be a large blue ball. Knock the ball at the Diva using the same method as the previous attack. This will take away a few more bars from her health energy.
Next, the Golden Diva will toss a hammer at you. Pick up the hammer and toss it in the air--NOT at the Golden Diva. Then, run under the hammer and it will hit you, turning you into Spring Wario. As Spring Wario, bounce up into the Diva's face.
Next, the Diva will send out small, explosive eggs. Pick up the eggs before they hit the ground. If they do hit the ground, they will hatch and turn into explosive ducks! When you pick up the egg, toss it immediately at the Diva before it explodes!
Her next attack will be a combination of all of the previous attacks. The attacks will come at random, so be prepared...
Finally, the Diva's face will transform into a hideous ugly monster. She will then try to smash you by stomping on the ground, breaking away the stone floor--which will reveal spikes. Attack her face directly [by pressing the B button]. After a few more hits, the Diva will explode and you will earn all of the treasures in the background!
Then the ending video will begin.
Wario will escape from the collapsing pyramid with the tiny black cat that always appears at the Boss levels. When outside the pyramid, the black cat will turn into either an ugly woman Wario or a beautiful princess--depending on how many of the boss treatures you managed to recieve.
Good luck--and have fun!
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