One Deus is defeated, its time to move on to Vlitra Core. Make sure to land counters when he rushes you, these are great boosts for your rage meter. This attack can take you from alive to dead instantly. During the last portion of the fight, where you're only Asura, he starts using a new attack that fires off lightning straight ahead of him, and he'll fly directly at you and land more melee hits as well as more lightning attacks. Dodge roll backwards multiple times to get out of this one. He has an AoE attack that causes lightning to strike around him going outwards in a circle. Look at for a few of Deus' attacks that can cause huge damage. The next two portions of the fight are harder because its harder to build up.
This is because the hits you land with the rapid fire (X button) are added up with any hits Yasha is landing with his rapid fire attacks. The first fight with Deus will be the easiest, as your Unlimited Gauge will fill up fairly quickly. Jumping leaves you open for more damage, whereas the dodge roll makes it so that while you're doing the dodge roll, you do not take damage for a second or two. Sometimes you can jump to avoid them, but this will not work at all times. Dodging his attacks is best often done with the dodge roll. By doing this, you can run in, get some hits, then back off. Stay at medium length will using the Jump+Rapid Fire to build up you're Unlimited meter. He has his electric nunchaku during this fight.
First as Asura with Yasha, then as Yasha, and then as just Asura. In you're next encounter, you've got a three part fight with him. Keep at it and eventually you'll have him. Watch for it, and then hold B to charge him, this will knock him in to the air, and when he hits the ground you can use a Y attack to inflict some extra damage. Like your first encounter with Yasha, Deus has a move you can watch for in which he takes a few extra seconds to recover. Your first encounter with Deus will be the easiest. When fighting bosses make sure if they hit you to try and get away as quick as possible, and use recovers to regain some lost health. The fight with Vlitra Core is actually easier than the fight with Deus. There will be spots that are very hard, including the fight with Deus (due mostly to how long it is). With the Mortal Gauge equipped, play through on Easy. Once you've collected your S Ranks, the game will inform you that you have unlocked the Mortal Gauge. I suggest playing on Easy, and using either the Destuctor Gauge or the Suvivor Gauge. To unlock the Mortal Gauge, you must finish 10 episodes with an S Rank.