In the image titled "Final explosion" is the flash the gun flash, or explosion flash? If it's the former, a delay between it and the explosion based on shell speed and distance might be nice.
I love the time speed up, but does that mean the stop watch won't make an appearance? It would look great to see the hand lift the watch into the screen, rather than being disembodied.
Letting it be hard makes sense, and letting the player know it should be hard sounds like a good solution. That also leaves it to the player to ignore the advice. I remember playing Arcanum and facing a pack of wolves at the start, and kept fighting them and losing. I eventually realized I was probably supposed to run from them, yet I kept fighting them all despite knowing I probably shouldn't.