This Secret Mission can be found during Main Mission 10, during the part where you’re introduced to the Hellbat enemy. You are exposed to timed doors in this mission, which are doors that look like they have teeth or spikes on them, and they will close once enough time passes. Should they close before you pass through them, the contents on the other side are lost until you replay the mission from the beginning (or go back to the last checkpoint). Anyway, when you are first introduced to the Hellbat, you will have a choice of two of these timed doors. Defeat the two waves of enemies in a quick manner, then run through the doorway that doesn’t have Red Orbs immediately behind it. Go all the way to the end of this path and stand on the raised platform in the middle, then look upwards to find the markings to begin this mission.
Depending on how you’ve done against the Death Scissors in the game thus far, this could be one of the easiest or hardest Secret Missions. You are required to defeat the Death Scissors in a single hit, which can be done by parrying their attack. Since the description says one shot, it might be wise to stick to your gun instead of chancing it with a melee weapon. Any normal damage done to the Death Scissor will result in you failing the mission. Back to the mission at hand, wait for the enemy to begin an attack animation (the forward cutting attack is the easiest one), swing your sword so it clashes with the scissors, then wait to see if some red mist comes out of its body. If it does, quickly fire your ranged weapon to instantly kill the Death Scissors.