The only achievement in this game that may give you some trouble (or carpal tunnel)
Remember, there are two keys to shoot, so alternating them quickly will allow you to fire much more quickly than just firing one gun. You can also change the controls on the launcher if some of your fingers get tired.
One tip that helped me was when I got tired around 10000 score, try to leave one land angel alive and you can just keep casually jumping and dodging its axe attack, while recovering hand energy.
I've included a video in case you need it: