Very nice game! (Although this is the first game I have ever rated...)
The game needs random variation like shooting time, robot bullet speed, robot accuracy. Just don't do any of this to the player.
You should create an invisible wall around the platform so you do not accidentally strafe off of it but that could certainly be a game mechanic.
E.g. a variation could be moving/shifting platforms.
I gave it the following: 3-4-2-4-4-5-3 (In order of categories.)