Star Fox 2/Gameplay

Difficulty
These are the main differences between the different difficulty levels you can choose.

Easy

 * You will have to destroy only two bases and two battleships.
 * Andross will send out only one space boss to fight you.
 * Andrew of Star Wolf will not appear.
 * The cores of battleships will only have two targets to destroy instead of four.
 * Brain Spoilers will not appear.
 * Battleships will sometimes fire the Planet Cannon.

Normal

 * You will have to destroy three bases and four battleships.
 * Andrew of Star Wolf will appear.
 * The cores of battleships will have four targets to destroy.
 * Brain Spoilers will appear.
 * Battleships will fire the Planet Cannon more often.

Expert

 * You will have to destroy six bases and four battleships.
 * Andross will send out two bosses to fight you.

With each difficulty level, the enemies you encounter will be harder to defeat, especially the bosses. Andross in particular will have one more battle phases for each difficulty level. Also, each planet level is different on each difficulty setting. For example, you only fight King Dodora on Hard (but not on Expert).