The Cobweb Cave.
Sebastian knew of killer moves, he's read and heard of killer moves.
Though he was someone whose sole attention was focused on his shark pets and anatomy in his previous life, he loved reading novels too to kill time. By reading novels, he read of killer moves.
He didn't know if any of his attackers was intelligent and genius enough to create a killer move, but he thought of this when he was still trapped and huddled up in the hole.
Despite thinking hard, he was not able to arrive at a solution to counter killer moves. That was why they were called killer moves, the variables were too much to speculate, it could be anything.
In the end, he resigned to embrace death if he faced one.
This was supposed to have been the case because the old toad's furious killer move would have cleanly wiped away his remaining health points without compromise, but another variable came into play.