Rank: A Mark: Adamantoise Cie'th Stone location: Sulyya Springs – Subterranean Lake Mark location: Archylte Steppe – Eastern Tors Reward: Genji Glove
From the moment you see what your mark is, you should know that this fight is going to be very, very painful. You've no doubt tried to kill Adamantoises in the field by now, only to get crushed like a pancake. Well, you're still pretty likely to get crushed, unless you act very, very quickly.
Summons are generally only really useful in specific situations, and this is one of them. Bring down your beast from the heavens, swap it into Gestalt mode, and then use its strongest attack to knock Adamantoise to the ground. Quickly put some buffs and debuffs on your team using Synergists and Saboteurs, but don't waste too much time with them – just throw Haste and a few attack buffs on – and then go to town. Once you've Staggered him, the fight will be much easier. Just pray you can off it before it manages to get up… which is easier said than done. However, another thing to note is that, like Neochu, he's vulnerable to Vanille's Death technique. It's easier to take him out this way once he's down and had a few debuffs stacked on him, but you're still going to have to rely heavily on luck this way.
Mission 64: The Doomherald
Rank: A Mark: Vercigetorix Cie'th Stone location: Oerba – Rust-eaten Bridge Mark location: Yaschas Massif – Paddraean Archaeopolis Reward: Gold Watch
This is it, guys – the last mark of the game. You can either do this the easy way, or the very hard way.
The easy way is to simply poison him and tank, waiting for his HP to dry up. This won't get you five stars, but it will get you the victory and the Trophies/Achievements. If you're up for "playing cheap," then this is the way to go.
If you're planning to tough it out, well, it's gonna be a rough ride. With over 15 million HP and resistances to almost everything, you're going to be spending a lot of time just chipping away, trying to Stagger him when the opportunity arises, Paradigm Shifting into defensive formations whenever necessary to recuperate. He has one particularly vicious attack that can slaughter you outright – if you see it coming, switch everyone into Sentinel mode and you'll lessen the damage by a great deal. Be constantly aware of your party's status and be ready to swap Paradigms immediately if things start looking bad. Again, patience and perseverance are a must – but once you beat this challenging foe, it will be one very sweet victory.
http://www.gamesradar.com/f/final-fantasy-xiii-cieth-stone-missions-guide/a-2010031011391394090/p-7 all info & credit goes to above website.