ZanacNES

Zanac is a vertically scrolling shoot-'em-up game that was released in November 1986. Take control of the state-of-the-art fighter, Zanac, with the objective of destroying the bases of the mechanized enemy forces. Use your full array of regular weapons as well as eight special weapons to clear the 12 intense areas of the game.

Be careful, though, as the game changes the degree of difficulty in real time based on how well you play. This ensures that it feels like you're experiencing a new game every time you play.