Cie'th Stone location: Faultwarrens – Sylphid Path/Gaian Path/Salamandrine Path/Nereid Path
Mark location: Faultwarrens – Titan's Throne
Reward: Genji Glove
Once you've conquered all of Titan's other challenges, you'll get to fight Attacus, one of the toughest foes in all of Final Fantasy XIII. He's got a whole bunch of HP, does very damaging attacks, is nearly impossible to Stagger (his percentile is 999.99%), and can cast Doom on your party leader when he gets down to 25% HP.
In other words, this is going to be a long and difficult fight you may have to retry several times.
Generally, Sentinels are among the least useful paradigms, but for this fight you should strongly consider bringing one in to help take the edge off of Attacus's most damaging strikes. Buffing/Debuffing via Synergists and Saboteurs is also a step you should take once the fight starts. For the most part, however, you'll just need to hold your ground and slowly chip away at his HP total. Ravagers will generally be more effective here than Commandos, so swap them in once you feel you've evened the odds enough. It's a long, tough fight, but with some patience and perseverance, you should be able to triumph in the end.