The more enemies you kill, the more weapons you unlock, and the more enemies that'll spawn (do keep in mind that the bots regularly launch into the air or towards you, and the mechs occasionally fly).
Don't Die
If you die, you lose everything, so make sure to watch your surroundings, otherwise you may get surrounded.
Save Your Progress
When you exit to main menu, your progress is saved for next time, but if you die or start a new game, that progress is deleted.