The long-range missiles from World 6 make a come back, and they are even more aggressive. This means you will not want to hang around too long on the screens that they appear in. This world is covered with a wide array of elevated highways and overpasses. King Kong can restructure their connections, and create new paths by punching the edges. This will allow him to reach certain portions of the world that would be otherwise unreachable.
You will most likely end up warping into the lower left corner of the world, even though the start point is located elsewhere. From this lower location, you should simply head east two screens. If your Rock Maximum is less than 90, collect the Rock Case. If, however, it is already 90, then you may not wish to collect any more Rock Cases, as this allows for your rock count to roll over from 99 to 0, and leave you with no rocks to throw. Go east one more screen to the boss, which you can only access from the lower left corner of the screen. Take it out and worry about collecting the Artificial Heart later. You should have a good collection of life points to defeat the boss with.
The boss of this world is a giant Bee that can send small Bee minions at you, as well as fire a laser based stinger. It will fly back and forth at high speeds along the top of the screen. If you stand mid-way along the very far right side of the room, you can remain safely out of danger, while you throw rocks that arc up to the Bee's location. As long as you have a good enough supply of rocks, you can defeat this boss easily. If you do run out of rocks, carefully position yourself just low enough to hit the Bee, but out of danger from getting hit. Defeat the Bee to collect the eight and final key.
After you defeat the boss, be sure to collect the Artificial Heart found in this world, before traveling to the upper left corner of the world and warping to World 9, the last world of the game.