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Before we begin, there are a couple of rules you should know and follow to be more proficient in battle:

1. With Nero, when using your "Buster" move against a boss, before using it, turn on Devil Trigger. Because of the animation used for the move, if you turn on Devil Trigger before you use Buster, and when the animation starts turn it off, you use a more powerful Buster move (as if you were in Devil Trigger) but do not drain your Devil Trigger. Also do not keep your gun charged during the animation, there is a little tweak that if you tap the gun button during a Devil Triggered Buster, that it shoots phantom swords adding a bit more free damage, so make sure to use that to your advantage.
2. When in a boss fight, or any fight for that matter, if you are in a hold where you cannot move or are taking constant damage for a prolonged period of time, turning on Devil Trigger provides about one second of invincibility where nothing hurts you, and you break out of an enemy's grasp.
3. During any battle with Nero, always have the "gun" button held down to keep it charging. For that I suggest only for Nero to change the button configuration from , to . As well as before any boss fight, max out Exceed by revving your sword, or if you have mastered Max Act and have purchased it, use that, although the Exceed does not play a major part, it just allows a couple stronger hits.

Now as you would guess, I am beginning with Nero's boss fights.

[edit] Dante Boss Fight

In either fight against Dante with Nero sure you could technically use a fair tactic, or you could use the easy way. With Nero against Dante you can just continually fire your gun (Blue Rose) then once you get close enough to him, Buster. This technique may be a tad difficult to get used to, due to the distance you need to be away from him, you can't be too far away, but if you get too close he uses his Stinger move.

[edit] Berial

In the battle against Berial you do not need much of a strategy until you reach DMD mode. It is basically dodge his attacks, attack him.

Here are some tips.

If you can, attack his head the most, if you attack his head it lowers the flames that engulf his body, if his flames are completely diminished, it leaves him vulnerable to be Bustered.

When Bustering, make sure to jump, and to, if you are able to, be in Devil Trigger so yes, follow golden rule #1 with no changes. The Buster even though he is still on the ground while you jump, counts as an in air Buster and does slightly more damage than a grounded Buster. (This seems to be in almost all cases that an in air Buster is stronger although there are some expectations.)

For his attack when he hurls a flaming ball into the earth and it creates pillars of flame that spawn beneath you, I would suggest to have practiced Table Hopper enough to get the timing correct as to avoid all 3 pillars (there has been a couple of circumstances on SoS mode and DMD mode that he has created 4 pillars). If you do not have Table Hopper or are just unable to perfect the timing, I suggest to jump backwards. Although I have not tried it myself, I have learned from personal experience that rolling to the side, does barely anything, it avoids the first pillar but almost guarantees you get hit by the second one.

My suggested move when attacking his head is Roulette Spin, as it does an efficient job at keeping you suspended in the air around his head while doing an ok amount of damage. Remember all the while using the strongest charge shot from Blue Rose as soon as it is charged.

If you keep up that strategy Berial, although he is a mediocre boss, will be a piece of cake.