Seymour is clever. It might take time, but he would figure out the weak spots. In the meantime, he would keep a tight defense. Making sure that Anima and the Mortibody had his back.

Once he finds these spots, bam, bam bam! Continuous strikes from his weapon. If Sigma catches him with a severe blow, he can fly away and Cura himself. The Mortibody could do it as well.

Say tha that little dog is quick and small. The Mortibody's strikes are quick and deadly. One or two strikes from Shattering Claw and his pet is dead.

If Velguader does deal leathal damage to Seymour, he can switch to defense. Let the Mortibody deal with the dog and Anima fight Sigma. Anima's blasts would be very powerful. Say Sigma dodges, his blasts still will blow the walls and pieces of the floor away.

All this time, while Anima and the Mortibody are fighting, Seymour can Cura himself. He can wait and plot a move. Wait for an opening and then he could strike. Perhaps while Sigma is busy in a duel with Anima, he could nail him on one of his weak spots with Flare.

Another thing he could do is make the Mortibody and Anima get Sigma and his pets or whatever together in one spot. Either nail them all down with a quick and powerful Ultima, or blow them and all the area around them to hell with Total Anihillation.

Sigma could teleport or fly away, but the Mortibody is still free. He would spot Sigma and nail him with Desperado.