Contra III: The Alien Wars/Stage 5: Mucho Grande Badlands

From StrategyWiki, the video game walkthrough and strategy guide wiki

The Mucho Grande Badlands stage is similar to stage 2 but quite a bit harder. Choose an area of the stage and get going. Be careful here since you can fall off the side pretty easily and the spinners near several of the bases can be disorienting. Press L button or R button to counteract the force of the spinning, depending on which direction they are going. After you destroy all of the bases, you'll face the boss.

Scorpellum: This thing is one of the hardest bosses in the game. The circular area around the boss will rotate like the spinners on the stage. Press L button or R button to stop the spinning. If you're playing on hard, the spinning will be very fast and you will need to double-tap L button or R button to turn around faster, although it is still harder to have a steady aim. The boss has two attacks of its own, a tentacle and a flamethrower. It doesn't use them too much but you should watch out for them. Keep yourself stabilized and fire at the eye of the boss. Use any Aura bombs that you have. Hopefully, you'll destroy it quickly and painlessly.