Sanctus

Sanctus Diabolica is the final boss of this game which is encountered in Mission 20.

Nero

At the start of the fight always move Nero back for a bit then Taunt to have enough DT-Gauge for the first cycle phase.
If you can MAX-Act pretty well then EX3-Calibur can instantly break the shield. However, keep in mind that getting MAX-Act all the time can be dangerous for optimal cycles.
If you plan on using Holy Water in Roulette Spins, make sure to give Sanctus some time after the 1st cycle.
The first time of shield breaking can give you ability to stop Sanctus from moving around instantly with Blue Rose shots.
To get Roulette Spins consistently your inital jump before the hit must be a long forward jump.
There is many variety tests against this boss and players have 3 options on how to kill the boss: Busterless, 1 Buster or Double.
These are the options to pick:

2 Buster - Recommended for new runners

Phase 1 : Showdown → Snatch → Buster
Phase 2 : Holy Water → Showdown → Snatch → Buster

1 Buster - If you failed to remain DT during the first buster then you are likely forced to buster for the second time

Phase 1 : Showdown → Snatch → Buster or Roulette Spin x3 → Snatch → Buster 
Phase 2 : Roulette Spin → Air Hike → Roulette Spin → Roulette Spin → Snatch → Roulette Spin till death

Busterless - A recipe for the disaster

Phase 1 : Roulette Spin → Air Hike → Roulette Spin → Roulette Spin → Snatch → Roulette Spin
After Ground : Roulette Spin x3 → Jump Away → Calibur → Aerial Combo [if the Calibur wasn't EX3]
Phase 2 : Roulette Spin → Air Hike → Roulette Spin → Roulette Spin → Snatch → Roulette Spin till death