Boron

Probably best known as the precursor to the genuinely loved sequel Boron II, this game is difficult because you are dead with a single hit. Just as in real life you get one shot at it. It is a shooter, but offers some challenging navigation elements as well, plus a pretty intimidating boss. It is unfortunately the first and final boss rolled into one. It's a short game which can likely be played in less than 2 minutes by someone who knows the foes and challenges of it.