In Numa Numa Trail, find Mateo without firing a bulletMission 5: Numa Numa Trail
At the very start of this mission, your objective will be to find Mateo. You don't have guns when you start but you can grab some if you want after killing the first enemy, however make sure to never fire a bullet until you reach Mateo.
Keep pressing to highligth the objective and to know where to go and to highlight enemies on your way. There aren't a lot of enemies in this section so if you stay outside of the main road it won't be too hard. You are also allowed to get spotted or kill enemies with your knife, if you really need to get rid of an enemy but it's really not necessary.