1. Stay in the middle of the map at all times. Don't even put your fingers on the Up/Down S/W Keys.
The enemies come from the top and bottom of your screen so just remain in the middle. If an enemy is going to hit you just move back or forward so you can kill them and move back in position.

2. Keep your mouse cursor close to the biker at all times. This will allow you to move it around quickly so that you can kill monsters that spawn closely above/below you.

3. Don't stare at the clock. It's tempting because you want to get to 150 seconds but just keep playing and don't worry about it because you will just get tense and make mistakes.