Not sure how I beat those Sir Tonberries - fight was kind of a mess.
All the post game fights have been a mess for me, since it devolves into me fighting alone since the party can't survive anything. For exceptionally tough hordes, there's no reason not to just wait for the free Ramuh summon to kill everything. It's how I beat most of Costlemark hordes at lvl 53ish, though the Jabberwock was pretty easy to take on with the party.