If I understand your comments correctly, it appears you're at the start of Episode 3, where the main story diverges into two paths:
Path 1 (Episode 3, Part 1): You went to Yaschas Massif -010 AF-, then to Oerba -200 AF-, and now you have a boss fight with Caius.
Path 2 (Episode 3, Part 2): You went to Sunleth Waterscape -300 AF- and met Snow.
Either path is viable and will require you to complete the other when you're finished: the strategy guide claims the second path is the harder one, but I found both to be about the same difficulty in my play-through.
Path 1 (Episode 3, Part 1)
The first fight with Caius is supposed to be a push-over: it's merely there to introduce him as an antagonist; he's toying with you.
If you're having difficulty taking him out, you're likely way too low level. Consider going back to earlier areas and completing fragment quests: getting all the fragments I could find and completing the first part of the Sunleth Waterscape -300 AF- (to the point where you get Moogle Throw) was more than enough to defeat him with a 5-star rating.
Path 2 (Episode 3, Part 2)
If you're trying to defeat the Royal Ripeness immediately after meeting Snow, you're going to have a bad time. Defeating it right after meeting Snow is an alternative route that you will not be able to complete until you're a bit more powerful (in the 2,000+ HP range).
Instead, follow Noel and Snow's advice, give the Royal Ripeness a wide berth, and head to the southern end of the map to complete the main storyline objectives.