Defeating Caius in Final Fantasy 13-2 will give you a headache. You'll get annoyed on how he beat your characters easily. On this guide you would be able to check some strategies here on how to beat Caius in Final Fantasy 13-2.
You will fight Caius five times on this game. So I will include five locations where you fought him and its strategy on how to beat this guy.
First Encounter (Oerba 200 AF)
Before the you fight him remember that Caius has auto-regen ability so it will automatically heal its HP. When the battle started first thing that you need to do is to have a Synergist in your party so you can buff all your party members. Then try to do a transition in COM/RAV/RAV or RAV/RAV/RAV in your party to stagger him quickly. Then once you staggered him change your paradigm into COM/COM/RAV. You need to attack him and maximize his stagger state. Then you will noticed that once his HP reached in the middle
he will cast Body and soul which will buff his stats. You can change your paradigm into COM/RAV/SAB to give him some debuff spells like deprotect, deshell and poison. Then use the same paradigm COM/COM/RAV to stagger him again. Don't forget also to change your paradigm into MED/MED/SEN if you think that your HP is in danger state.
Second Encounter (The Void Beyond - Year Unknown)
You need to focus staggering Caius on this battle. RAV/COM or RAV/RAV to stagger him quickly. YOu can also have SYN/RAV if you have chance to buff your party. Then once you staggered him use COM/COM to deal more damage. If you see that he will use Blast Wave, switch into MED/MED to heal your party, because this attack will give you serious damage to your HP.
You will notice that if he used Chain Break all the chains of your stagger will break. If this happens use RAV/RAV to try to stagger him again. There's another thing that you need to be careful The pulsar burst. If this one hits you, just change your party into RAV/MED so at while your casting magic on him at the same time someone heals you.
Third Encounter (A Dying World - 700 AF)
This fight is one on one battle against Caius and you'll use noel only. At the start of the battle Caius will launch noel up in the air. So it will give him time to cast Heart of Chaos. Just use the normal COM paradigm and attack Caius. Same with other battle with him, once you've reached 50% of his HP. He will cast Body and Soul to buff his battle stats. If he keeps mashing you and you don't have time to change MED paradigm. It would be faster if you'll use items like potion or hi-potion.
If you are defeated, no worries because it will only continue the story. You can defeat him using this strategy but you need to consider also your level and equipment.
Fourth Encounter (Academia - 500 AF)
In this area Caius has two forms, Chaos Bahamut and Caius. His default form is Chaos Bahamut in the first set of battle and for the second set after you beat him he will transform into Caius. Use RAV/RAV/COM on this battle to stagger him. Every time he attacks your party or your health gauge reduced by 50%. Use MED/MED/MED to heal them then change it into RAV/RAV/COM. If Chaos Bahamut will use Megaflare. Change your paradigm into SEN/SEN/SEN. Then use MED/MED/MED to heal all your party members.
After you beat Chaos Bahamut, he will turn into normal Caius. Once the battle started he will cast Eye of the Bahamut to one character. This will give your character a bunch of debuff spells like poison, slow, deprotect and deshell so it would be better if you have a Synergist and Medic in your party to remove this debuff spells and give your players buff spells like protect, shell and haste. Keep an eye on Eye of the Bahamut because sometimes in the middle of your battle he will use it. Same strategy applies here, stagger him using RAV/RAV/COM then if he is on stagger state change the paradigm into COM/COM/COM. This guy always use Chain break and Body and Soul so prepare for that.
Fifth Encounter (Valhalla - ??? AF)
At the start of the battle he will cast Body and Soul to buff himself. Then sometimes he will cast Ultima. If this happens change your paradigm into MED/SAB/SEN. After it heals the party change it into RAV/RAV/COM to build the stagger gauge. If you staggered him change the paradigm into COM/COM/RAV to deal big damage whie he still in stagger state. After you beat this guy he will revive automatically. This time he is faster and he will execute more powerful attacks like Giga-Graviton and Pulsar Burst. And Eye of the Bahamut. Always check your HP, if you received huge damage, use MED/MED/SEN to restore your HP and removed status effect if you've been hit by Eye of the Bahamut. If you have another chance to stagger him, quickly change the paradigm into COM/COM/COM for easy kill.
You can follow the guide here or you can mix your own strategy to beat him. I used this strategy on how to beat this one. So it works for me very well.