I said it a couple of times. He can not only use that wird weapon that he used in the battle against SinSpawn Gui, but he can use the Lance of Atrophy. The Lance of Atrophy is a huge weapon.
This weapon's mass, combined with Seymour's extrordinary strengh would make all to easy for him to pierce Sigma's armor.


And say that didn't work. He could just start break the Funhouse and have its structure come crashing down on Sigma. This would also work if Sigma had put himself into the Funhouse's structure as Senesia said he could.