Leveling up is slow, yet important. Each time you level up you have a chance at increasing your lives...and lives are the only way to win. Each level after stage 2, requires you to be a bullet sponge. It was fun at first until you realize there is no real strategy invloved in order to win. You must get lucky, grind for leveling (making it a few stages each game) and or watch ads to retry/revive.