The Ectopudding is an interesting enemy. LightningMagic is basically the only way to really hurt it, and it's not terribly vulnerable to any Status Ailments. However, its very low chain resistance means its chain gauge fills up extremely fast, especially with a paradigm like Relentless Assault. You can expect to stagger this foe before killing it, despite the 600% stagger point. Just be careful of its very powerful attacks, healing or using a Sentinel as necessary.