I liked how the shield system was implemented. The enemies that you shoot for oxygen feel like you can only reasonably shoot them head on, and because they're shooting forward you have to use your shield to walk in front of them to shoot at them and get oxygen. It's like the cooldown of your shield is a resource where you have to decide if you want to use it to position yourself for the boss or use it refill your depleting oxygen. The choice between upgrades makes it even more fun because you essentially get to choose how you want to prioritize your time between the boss and oxygen management. Overall all of the systems are really well thought out with how they all interact with eachother to make a complete product. Keep up the good work!