So you're saying that Sigma could use the traps and Seymour couldn't? Seymour is far from stupid. He is an expert manipulater and would definately find a way to use the traps to his advantage.

So, he has elemental defense. Flare, Total Anihilation and Ultima, not to mention Desperado, are all non-elemental attacks. Seymour would soon determine his attacks would not penatrate Sigma's armor and then would find a vulnerable spot. He's very calculating. He wouldn't continue attacking Sigma without figuring out his weak spots.

As for Sigma's extra parts, that's what the Mortibody is for. Guarding Seymour while he attacks. He could blast them apart with Desperado or cut them into pieces with Shattering Claw.

As I mentioned, he can fly. He could fly around and dodge Sigma's lasers attacks or whatever.

If the worst came to worst, he could flee in his normal form. He would most certainly have a back-up plan to cover him. Perhaps a giant whole in the floor or a giant bolder or whatever to distract Sigma, giving him the advantage. He could strike Simga in his vulnerable spots with his weapon why Sigma was preoccupied.

What makes you think that Seymour would fight Sigma directly? He's no fool. He could just wait and plan his moves. Attacking head-on is not always a wise idea, and Seymour would definately know that.