In The Dark Walkthrough
Here are some tips:
The main challenge of the game is to kill all enemies fast. If you let them walk around, you will not be able to get them all before they reach the bottom.
If an enemy hits the bottom, you lose 5 hp. That's a lot.
Use the slow motion by pressing SPACE. Its much easier to think and aim when things are going slower ;)
If you deplete your whole slow motion bar, you will have to wait until its full to use it again. Try stopping just before the bar is empty.
Upgrading your amount of bullets per shot at the beginning of the game is very effective.
Upgrading the grenades quantity at the beginning will help you later in the game as you will need a lot of them.
Don't forget to upgrade your health regeneration rate. It can save you from a mistake.