To unlock the Mortal Gauge, you must finish 10 episodes with an S Rank. These have to be on the SAME difficulty. Not 10 episodes across difficulties. I suggest playing on Easy, and using either the Destuctor Gauge or the Suvivor Gauge. Once you've collected your S Ranks, the game will inform you that you have unlocked the Mortal Gauge.
With the Mortal Gauge equipped, play through on Easy. There will be spots that are very hard, including the fight with Deus (due mostly to how long it is). The fight with Vlitra Core is actually easier than the fight with Deus.
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.
Your first encounter with Deus will be the easiest. Although he is still hard. Like your first encounter with Yasha, Deus has a move you can watch for in which he takes a few extra seconds to recover. 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. Keep at it and eventually you'll have him.
In you're next encounter, you've got a three part fight with him. First as Asura with Yasha, then as Yasha, and then as just Asura. He has his electric nunchaku during this fight. Stay at medium length will using the Jump+Rapid Fire to build up you're Unlimited meter. By doing this, you can run in, get some hits, then back off. Dodging his attacks is best often done with the dodge roll. Sometimes you can jump to avoid them, but this will not work at all times. 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.
The first fight with Deus will be the easiest, as your Unlimited Gauge will fill up fairly quickly. 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 next two portions of the fight are harder because its harder to build up.
Look at for a few of Deus' attacks that can cause huge damage. He has an AoE attack that causes lightning to strike around him going outwards in a circle. Dodge roll backwards multiple times to get out of this one. 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. WATCH OUT FOR THIS ONE AND DODGE IT. It can, and probably will, hit you. This attack can take you from alive to dead instantly.
Make sure to land counters when he rushes you, these are great boosts for your rage meter. One Deus is defeated, its time to move on to Vlitra Core.
You have to beat Episode 18 and True Episode 18 to get the achievement, so be aware that you have to fight and beat Vlitra Core twice.
Vlitra Core is easier than Deus, but he has plenty of moves that can do major damage when they hit you. When the fight first starts, start running to the right. Make sure to get away from Yasha. VC's first attack is the attack he uses to fly at you. If you can dodge it, you'll keep half your health bar that would otherwise be lost.
Unlike Deus, VC won't knock away or dodge your rapid fire, so make sure to keep using X to build up your Unlimited bar. When thats full, rush in, land a few combos, then a heavy. Repeat this process until you're unlimited bar is gone, then back off. Your rage bar should have a nice little bonus on this. Most of this fight is doing this, and then backing off and dodging VC's attacks.
He has two attacks that require button presses to dodge. The first one is where he throws his entire body at you. This one is easy to spot and easy to dodge. The camera will focus on VC and he'll roar and disappear. Wait for the signal, then press the Y button (On easier difficulties, I noticed this switches between Y and B, however, with this gauge active, I never had it show B as a button press. But be wary, it could possibly be the B button). His second one is where he throws a spire of lava at you. This one is a bit harder to deal with. Watch the signal, because it alternates between the Y button and the B button to dodge. If you miss it, this will take half of your health bar, at least. The build up to this attack is seeing VC warp away from you and Yasha. He'll then attack you with a long range area of effect lava attack (similar to the one I mention below), dodge out of it, and he'll use to red swords and sweep the field with both of them. Dodge those, and he'll build up this lava and throw it at you.
VC also has a red sword that he'll occasionally try to hit you with. When you see his hands go up in to the air, start dodging right away. It will either be the red sword or a area of effect lava attack, similar to the one mentioned above, except its short range, not long range. Dodge roll out of it, and keep going. When you're rage bar is built up, do not activate it right away, unless your health is very low. Its rare, but on occasion you'll activate rage right as VC warps to the other side of the field to do his combo short range AoE/field Sweep. If this happens, you're rage bar will go down and you'll have to keep fighting.
With both Deus and Vlitra Core, I stress the use of patience, as well. Most of my failures came from trying to rush the battle. Deus and VC together easily can take an hour and a half to two hours of work.
Best of luck, and I hope this guide helps.