SMN is only a hindrance now. The rarab tail lowers his defense and now leaves him terrorized for a minute.
SMN is NEVER, EVER a hinderance in that fight. SMN increases the damage output of your party by far more in that fight than adding a DD in its place.

Screw the rarab tail. It's a waste of time to obtain it when the fight is perfectly beatable without it. The first time we ran it, we considered getting them, but nobody wanted to wait for it.