Ambling Bellows is a mark in Final Fantasy XIII, found in Taejin's Tower, as part of Mission 22, Mission 18 and Mission 10. Similar to the related Boxed Phalanx enemy, it can call and power up the Cryptos, and is a formidable foe on its own as well. It will call more Cryptoses if they are all destroyed, but will not if one still remains.
A quick way to defeat it is to gain a Preemptive Strike, which almost fills up the chain gauge. A single strike should be enough to Stagger it, whereupon it can be launched and quickly defeated. Failing this, you could fight until HP is low, and then summon an Eidolon, filling your health and allowing the monster to be more easily defeated.