Oh, that. The Requiem DLC consist of two battles. You need to win both battles with a 5-Star-Ranking.
To do so, try to bring Lightning to Level 7 or 8 and you have a fair chance to beat the second fight with five stars. I had a little strategy, let's see if I can remember.
First of all, start by casting the defensive Buff-Spell as Synergist. Though the offensive one grants you haste, I consider the defensive one to be better. Can't remember the name. Sadly. Start your attack by using a physical attack. Then switch to the first Magicbased role. Attack, then after the second attack switch to the second magician role. This role will have its ATB filled by a lot, so don't think twice and use it right away. Switch back to the first magical role, repeat. This strategy is best used, while Caius has been staggered or is acting as a medic. Light's Ravager roles also inflict bleed damage, so Caius's healing won't help him much anymore. If Caius goes Commander, you might get two attacks in, before he announces his attack. Switch to Knight ASAP and use the guard spell. He'll attack once, twice if he's at half health. If caius goes Ravager... well, I have no idea what would be best, as I can't make out his moves. If the defensive buff is in effect, you can't be interrupted anymore, so go all out. You will take damage though. So switch to Knight, if Caius casts those purple energy balls.
Once he is staggered, switch to Paladin and sent him airborn. Then switch back to the Ravager roles. You can keep Caius in the air forever (while staggered), by simply switching between both Ravager-Roles, once the last spell has been cast. Before his staggering ends, switch back to Paladin and use Army of One. This will heal you and the more damage you do, the more is healed. Obviously, if the stagger gauge is very high, Lightning can heal to full. If there's still time left, end the staggering with one of the Paladin's attacks, to inflict massive damage. Repeat the process. Oh, for extra bonus, use your Saboteur-Class to inflict some status ailments on him. Oh and if you notice that the stagger bar sinks quicker, throw a paladin attack in.
As for round 2, Caius can now ONLY be inflicted by status ailments, if he has been staggered. Your strategy hasn't changed much. Attack regular, then switch between both Ravager-Classes. but beware, after some time, Caius will be immun to either wind or lightning damage. The stagger bar can still be filled, but avoid the class, Caius is immune to now. Another thing is, once Caius starts to transform, he'll hit very hard. If he does, switch to Knight. Block his attack, then switch to an offensive role, attack and switch back to knight. Once he has used Ultima or Megaflare(?), he'll change back to normal and you can use your regular strategy. Once you stagger your enemy, cast a debuff on Caius. Again, if the staggering ends, use Army of One, be a Paladin and attack, nefore the stagger ends, so you can inflict a lot of damage. Repeat. Oh, and cast the defensive buff for this one as well.
I hope my little help doesn't sound to confusing, I'm no native english speaker. But you'll figure it out, once you try to follow my advise. It's not that hard if you what's to do.
I'm glad I could be of help. I'm afraid, you need to observe the second fight first, before you know what to do. For example, Caius will switch into two different "forms" in his second form. Depending on his immunity. This also changes whether he uses Ultima or Megaflare(?). And his immunity is removed, once he transforms back to normal.
I also forgot, in phase two you can't send Caius into the air anymore.
Tell me how well it worked for you, once you managed to unlock lightning.
also, make sure you're in good health once you defeat Caius. Being at good health raises your points and you receive a good ranking muc easier. So, if you know Caius will die in a few hits, use that Elixier in round 2. Only use Army of One for healing. With a proper level, that won't be too hard.
It has been 5 starred at level 0 and can easily be done at level 5 when you get Aura.
Use magic against Commando stance, use physical attacks against Ravager stance. Tank with whatever the equivalent of Sentinel is even if you don't get Immortality up in time. Use your potions and elixir wisely. It's... not that hard.