nextWAVE | A project for HackIllinois

I am a beginner in Libgdx and I have a simple Galaga type game setup where the player earns points through waves with different level enemies with various stats. The player can then upgrade certain ship stats with these points. Mostly everything is done all I am focused on now is making the gameplay balanced between player ship stats and the different enemy stats as the player progresses. I want the game to be infinite as in the player can go on for as long as they can last but I can't seem to figure out how to set up the enemy spawning so that as the player progresses the enemies have different/harder stats and there are more enemies.

Here is my spawnEnemies method in my GameScreen class which adds the EnemyShip object to an array that is iterated through and then each ship is rendered in the render method.