Ok, first boss down, key acquired. Now I can explore new areas, yay! Which I did, and was promptly killed. Note to self: you can't duck the plant bullets.
Boss spoiler: GIANT ENEMY CRAB! Ok, hes not that giant but he is larger than your average crab. And he lives in a rock. I was having trouble killing him because I'd run out of shots and hes hard as hell to hit with grenades. So protip: throw a grenade or two at him before you grab the key. They will explode when he activates and deal a ton of damage right from the start. He didn't even get to drop boulders on my head before I killed him.
Hoping theres some health upgrade to find soon.