Considering he is fast, smart and can fly plus teleport, it makes it much more challenging.
Still Seymour's physical defense plus Protect would reduce Sigma's damage to him. Not to mention Seymour or the Mortibody's Cura, would further negate the attacks.
If he does transform into that huge form, then the destruction of the area around him would distabalize him. Why he's blundering around Seymour can strike with Multi-Thundera and Thunderga. And I guess Flare.
Mrtibody's Desperado could keep him back and allow Seymour to keep hitting him with his magic attacks.
Contrary to what people think, Seymour is quite physically powerful. I mean he held off a huge Sinspawn with his bare hands.
He has that weird weapon he uses in the Sinspawn Gui battle(2). He could cast Protect and strike Sigma with it. He would be slightly quicker with his half-human half-guado body.
If that proved to be a bust he can transform back into Omnis or Seymour X form. With the wings given to him in Flux form, he can fly around and attack Sigma. It might in fact come down to a battle in the air.
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