If you don't want to wait for your energy to refill just exit the game, go to the time settings on your phone and forward the time for 60 minutes and you'll have extra energy when you get back in the game.

Headshots are not only sweet but lucrative too as they earn you extra coins so you have an additional reason to aim for the head.

Snipers are able to stay still for insanely long periods of time, so you should also practice patience as the mission will sometimes require you to, wait for the "pink mist", the perfect shot.