This was quite fun. It was indeed short, as expected.
I didn't like how it suddenly became a lot harder to move around while shooting. It's a good idea, but if there's going to be more shooting in the full game, it's going to get really frustrating rather soon, especially if faster enemies are introduced. I'd suggest some sort of animation to show when the character is going to be moving slow and firing, so players can either learn the timing before they return to full speed, or something like that.
The final boss was fun, fairly clever, but I thought it drug on just a little too long, especially once I got to where I could kill the split off mini-ghosts quickly (despite the aforementioned slow moving while shooting)
I do like a bit more puzzle in my boss fights, but that's just personal preference, and you seemed to be doing okay (albeit a little easy) with the puzzles outside the boss fight.
Oh, and the switch that needed the grenade to be hit was a little irritating, but that's probably due to me trying to throw grenades while jumping, before realizing that it couldn't be done.