• A loaded flare gun.
  
• A shotgun loaded with at least one shell (But preferably two).
  
• A large room.

So you have the stuff, next you need the monkey. Crawl slowly in the direction of the snoring until you see it at the edge of your screen. Position yourself so the monkey is in an intercardinal[en.wiktionary.org] direction from you and there is a large space behind it. Unholster your flare gun, make sure you're more than an arm-length away and shoot it.

The monkey should run towards you for a moment, then run towards the flare that just shot through it. Take this moment while it's chasing the flare to holster your flare gun and pull out your shotgun. The monkey will start to roar and run in circles. If your flare did not travel far enough it may find you. If this happens, shoot it with the shotgun. If it survives it will rip you limb from limb, so you may need to try a few times. Remember to shoot the flare from an intercardinal[en.wiktionary.org] direction.

Once it stops howling, wait ten seconds or so for it to go back to idling. Holster your shotgun, crawl in the direction of your flare's movement and follow the snoring. Take out your shotgun and put a shell in it, it should die in one hit. If you don't want to go looking for it, wait for it to stop roaring and fire a shell in its general direction. It'll come find you.

Hooray! You have killed a monkey! Celebrate by using your many loud guns with impunity.