Sigma doesn't even have to show his core all the time. He can possess the arena and have his body parts to attack. Of course he cannot do this indefinitely, but Sigma will have a lot of time in his hands to study Seymour's movement and attack.

As I have mentioned a lot of times, attack such as Ultima and Flare won't work effectly against Sigma. (Zero's Gaia attack doesn't work...X's Nova Strike would work, but that's because Nova strike is a body smashing, pin-pointing attack which requires aiming.)

There is one thing though...can Seymour have Mortibody and Anima at the same time? I mean... I don't think I've seen that.

But then Anima isn't too powerful. Pain will not cause an instant KO to Sigma, and will result in minor damage. Due to the size of Anima and Mortibody, it will be hard for them to dodge Sigma's attack, and they won't last too long. Mortibody is big and predictable, it is not hard to see when it will use his attack.