Frustrating
I beat it and got in the top ten today. So I was pretty good at it.
But even then, I think there were a couple points where the difficulty was a bit much. The bubble split in a way that was more cheap than just difficult, and after the spinning death room in Mission 13, the wall that you need to blow up had a very weird shape that made the point you had to shoot unknown. If you didn't hit in the random spot, you'd be forced to take damage passing through a remaining part of the wall.
But it wasn't so hard I couldn't beat it, and beat it well. I enjoyed being angry at the game.