Level Up!

In this side-scrolling platformer/shooter with finely destructible levels and bosses you level up your character by literally going up to the next level. Each level-up earns you some points to spend on upgrades: more health, firepower, new weapons, skills, etc. Also it's made of voxels, so you can destroy walls and dig tunnels or trenches to hide from enemy fire, and blast holes in huge bosses to get to the vulnerable spots inside them.