I got to the (first?) Caius fight and i am having a rough time beating him. Every time I get him below 50% health he starts kicking my ass, and regening his health rapidly.

Caius has a technique that lets him regain health if he's left alone. If you keep pressing the attack, he won't be able to finish getting it off. The way I took him down in that fight was with Relentless Assault, Delta Attack, Potions, and only switching to a Paradigm with a medic/sentinels when absolutely necessary or I when I needed to guard against one of his more powerful attacks. Basically, you need to press the attack on him as hard and as fast as you can. Boosting your characters will also help.

Don't bother. Make wound damage your best friend for that fight. After doing as much damage as you can during a stagger switch to Sab/Sab and toss whatever you can at him. Any healing he does will be almost pointless. Oh also dispel his buffs, that is also key. Basically Sabs in general are key for everything.