The fourth level within the Russian scientist's castle begins with a drop through one screen (where you can grab item refills if you like), then into the next screen. There, you'll see two cannons sitting on ledges of varying height. You can take out those cannons.
Of more interest is the energy tank sitting in a recess beneath the second cannon. You can drop down and press against the wall to fall just to the side of a barrier, which can be destroyed with the drill missile so that you can gain access to the energy tank. Once you have done so, go ahead and drop down to the next screen.
From the bottom, head toward the right and you will find one of the chubby pogo sticks. Defeat it and continue to the right. You'll see a series of ledges you can slide under, but make sure you do so after one of the electrical charges so that you are not damaged on the way through.
On the other side of that red pipe, continue right to the next red pipe, elevated on a low, gray ledge. From there, continue along the path to the right. Next climb up the ladders into the next room. Here, the path can branch. You can either head left up the ladder to the upper left, or you can use your Wire shot to reach the high green barrier, and drop down the pit to the right side of the screen to take that route.
I'll assume you take the path down the pit to the right, which leads to an energy tank. When you fall down the pit, head left and you'll see another green barrier between you and an energy tank. Use the drill missile to destroy the barrier, grab the tank, then drop down the ladder to the floor below. Work your way to the right along the area, and shoot the slinky robots as they descend the steps. Past those, you'll pass into the next screen.
Here, red-shielded enemies are zipping back and forth over a series of red pipes. This is where you would have emerged if you took the other path. Pass beneath those and continue toward the right. You'll see an upper path and a lower path here. If you take the upper path, you can move past another red-shielded enemy to grab a large energy capsule, while you can take the lower one to sneak past some electrical charges. Either way, you'll drop down a pit on the right side of the screen to the area below.
The next room is devoid of enemies, so just run down the steps to the left and drop to the next area. Here, you'll see another tube that drops the skull-shaped robots. Destroy them and move toward the right, past a second tube, to the door where you'll finally get to fight Dr. Cossack.
NEXT: Dr. Wily's Castle