in order to expose it's weak point, you must attack at the same time the enemy does. There will be a visual cue when it does: the scissors will slightly glow white. When it enters it's attack animation, you must stinger into it. The easiest attack to parry is the one where it rolls around with it's scissors. If stinger isn't working, try to use different attacks to parry.

after a successful parry, the enemy's head will have a red aura surround it, this is it's weak point exposed
Now all that's left to do is to shoot at it, this will complete the mission and reward you with a blue orb fragment

This is it for my guide, it was very short and simple, this can also be used as a tutorial to easily defeat the death scissors enemy. My next dmc5 guide will be an easy method for secret mission 6.