I used a really cheap strategy, but it worked wonders! What I did was getting a decent SEN monster, in my case it was one of those mecha robots you get in the Augusta Tower, and maxed him out (early peaker); he ended up with about 6500 HP. Afterwards I went into that fight with Serah as SAB, and the SEN monster in all my paradigms, this way while the SEN monster got all the attention and damage from Caius, Serah debuffed and dealt wound damage to Caius which is very useful since his HP constantly regenerates. After about 10 minutes his max HP got reduced to 27009, which appears to be his minimum max. HP (
http://i.imgur.com/HX1ut.jpg), at which point a barrage of spells with Serah as COM or RAV should be enough. Obviously have a MED/SEN paradigm for when its needed, and having the SEN monster know Mediguard helps a lot too.