Seymour is immune to silence.
The best tactic is to train, It might be boring but Seymour Mortis is the least of your problems if you only have one character above 3000 HP.
My characters were pretty lame too. Just run around by the save point to train up and keep saving to replenish H/MP.
Full overdrives are really important and so are aeon overdrives. And constant healing.
good luck mate, keep us posted!