In this mission, you have to defeat all of the enemies with NO TIME LIMIT. There are 3 white Arachnes, 2 green Arachnes and 2 Hell Vanguards.
This one can be found at one of the lights glowing red in the Spiral Staircase in Mission 13.
For first timers:
This could be hard, but you just need Agni & Rudra, Ebony & Ivory and Shotgun. You can shoot afar, but watch out for Hell Vanguard's attacks when ripples appear beneath your feet, or when the church bell rings. The same is true with Arachnes, so you have to deal first with Arachnes using your Agni & Rudra.
For experienced players:
Use Kalina Ann and Beowulf's Volcano. Shoot and bust a lot. You can deal first with Hell Vanguards with Beowulf combos, then the Arachnes with Agni & Rudra or Cerberus. Using Twister or Ice Age may help a lot clearing Arachnes.
If you are fighting with a Hell Vanguard, use your Beowulf. When you parry with each other, use Trick Up when it is about to hit you and mash . Then perform Lunar Phase and continue attacking until you parry with each other again.