Wow, the juice is real in this game. I really like the idea and the presentation, so don't take this list of criticism to mean I disliked it, but rather that I cared about it enough to put some thought into how it could be improved. :)
The physics are kinda janky. I often shoot a bullet at a surface at an angle and it comes straight back at me. It looks like what happens is that the bullet penetrates into the surface and detects both a horizontal and vertical collision and then comes straight back. That means some levels I just had to redo a few times until the shot happened to work.
The aiming is a bit imprecise. If I put the cursor exactly on the corner of a wall (e.g. the corner that the opponent shoots in the first level with diagonals), the bullet won't actually hit that corner, but either go past the wall or hit it away from the corner.
There's no clear indication of when the second is over. Since I'm probably looking at the cursor and level layout to do my aiming, I don't really want to be checking the timer for when I can click, so I just start mashing the button once I'm done aiming. There could be some kind of whole-screen indicator that the timer is complete, e.g. a flash or a change of the colour scheme.
The fact that the levels are static means that if you fail them, the 1-second appeal is kinda lost, because now you've had X seconds to stare at the level and make a plan. At that point it just becomes trial and error to figure out the right shot and is no longer about quickly judging a situation. This could be fixed by randomising the levels to some degree, but probably only small variations so you can still control the theme and difficulty of each level.
The difficulty curve is off. The diagonal levels in the (early?) middle are definitely the hardest, and afterwards it gets very easy again.
You could escalate the difficulty further by relaxing the "only one" theme on the enemy count and having your bullets pierce enemies, so you'd quickly have to find a shot that goes through multiple enemies.
Again, I really liked it and would love to see a more polished version. Keep it up!