Here are some tips on fighting Sanctus:
1. Sanctus has barrier that protects him. It is a priority to destroy the barrier in order to damage Sanctus. Don't bother using Blue Rose or Buster as the it has no effect on the barrier. Instead, use Red Queen. Aerial Combo and Roulette Spin should be used to destroy the barrier. Roulette Spin is more useful as it has multiple hits, and it can easily destroy Sanctus's barrier.
2. Sanctus's fireballs attack are weak, but fast enough to knock Nero down. One way is to just use Side Roll, Air Hike, or Table Hopper to evade the fireballs. You can even shoot the fireballs with Blue Rose, but only when you are locking on him.
3. His random lasers attack are powerful, and it is hard to predict where it will hit. Whenever you see he is starting to use the attack, just run away, and don't bother attacking Sanctus.
4. His flame waves attack are powerful, but can be easily dodged as it only hits the ground. Jump and use Snatch on Sanctus to get close on him and attack him using Red Queen attacks.
5. The two objects floating can be used as to reach Sanctus quickly. Use Hell Bound/Snatch on it to get close to Sanctus. Be aware that those objects can become hostile and attack Nero with minor damage.
6. His lightning wave attack is powerful too, but it only hits the ground. Jump and use Roulette Spin or Aerial Combo to bring Sanctus down or destroy his barrier.
7. Once Sanctus's barrier destroyed, there is chance to bring him down defenseless by repeated Aerial Combo or Roulette Spin attacks. Do it quickly before he uses the barrier back. Once he is on the ground, use any attacks to damage him, but preferably Red Queen Combo C in D.T. mode, then Devil Buster on D.T. mode too. Showdown or Maximum Bet works too, but it takes time. You can use Summoned Swords while Nero is using Buster or Devil Buster to damage him further. Please note that after using Buster or Devil Buster, Sanctus will always summon his barrier, so make sure the Buster or Devil Buster as the finishing combo whenever he is defenseless on the ground.
To be honest, Sanctus fight is not hard, but annoying. During the second phase of the battle, he will try to run away from Nero after his barrier is destroyed and will quickly got to Savior to save himself. You must get to him quickly after his barrier gone during second phase.
