Collect Health Powerups until you have 15 HP, then collect an Attack Powerup to attack the Reaper. Your mouse is your character.
You may have heard of the Grim Reaper, the specter who takes the lives of those due to die. What you may not know is that the Grim Reaper is not one person, but rather a large group of Reapers. You have challenged one of these Reapers to a wager: You do battle with him, and the winner becomes/remains a Reaper and gains the other's power. The loser... dies. It's a fight for your life, so be careful not to die! (The project this is based on is unrelated to this one in all but concept and inspiration. Give it a try anyways!)