One way to do this would be to have an instance of each enemy type initialized within the class that handles spawning. Something along the lines of:
public Enemy lizard;
public Enemy fox;
That will expose those parameters to the inspector, so you can just drop your prefabs in. From there, one solution would be to initialize each level in it’s own array:
private Enemy [] easyDifficulty = {lizard};
private Enemy [] mediumDifficulty = {lizard, fox};
You can then have a “currentDifficulty” array that you handle spawning from, and change its value whenever the difficulty changes.
// Initial assignment to default
private Enemy [] currentDifficulty = mediumDifficulty;
// Potential method for changing difficulty
public void ChangeDifficulty (int difficulty) {
switch (difficulty) {
case 1:
currentDifficulty = easyDifficulty;
break;
case 2:
currentDifficulty = mediumDifficulty;
break;
}
}