Fiend Arena Oversoul
Despite its large HP and Defense, it is an easy enemy to defeat. Its sole physical attack deals fractional damage equal to 31.25% of the character's current HP, meaning it cannot kill that character. It becomes more dangerous when Oversouled as it gains the ability to use Demi and Slurp. Demi cannot kill characters, but Slurp can.
Shell Shocker can be recruited into the party anytime during the game by setting up a Trap pod L in the Moonflow. It comes into the party having learned Ailment Def. and Total Limit Break and has extremely high HP and Defense even at low levels, making it one of the most powerful fiends that can be captured in Chapter 1.
One notable Shell Shocker who was captured by the Gullwings constantly whines about lacking energy and motivation. It gets so obnoxious that Shinra finally marches the lazy snail up the Mi'ihen Highroad, intent on banishing the Shell Shocker to the Farplane. Viewing this Fiend Tale ending unlocks the Sleepy Sneaky team in the Fiend Arena.
The Shell Shocker is much stronger in the Fiend Arena, as it now has access to Meteor, which can be devastating to a low-leveled party.