Overall game is nice :D
The jumps feel a bit 'floaty' though but it blends well.
About the Grim Reaper part, it would have been a bit better if the player spawns back to the position where he left before fighting the Grim Reaper or maybe add some Checkpoint type system, because while it makes sense to start at the beginning due to the limitation, it seems like an extra bit, because you have to replay from the start anyway. But it is understandable why it has been put, and it fits the limitation well :)
As for the gameplay, the fight with the Grim Reaper is interesting initially, but afterwards it is easy to figure out how to defeat him [Just wait at one position and keep shooting, and jump when The Grim Reaper shoots at you]. One thing that can be done here is to add a dynamic AI for this which can possibly add to the gameplay, for example changing how the Grim Reaper attacks you, or maybe add a few platforms through which you can jump, as well as the Grim Reaper, and maybe you can hide under some platforms to protect yourself, after which the Grim Reaper changes his position, and so on. This, or something similar, can potentially increase the gameplay, while at the same time increase the player's engagement. If you are interested, you can try out my game, I tried to keep the gameplay interesting as much as possible :)