Not really a solution, but I wanted to comment on an alternate method of unlocking this that I stumbled upon.
Normally you'd want to beat the campaign without ever retreating from battle.
It turns out, though, that Arena Mode also counts for this. And you can't really retreat in that mode - it's a series of around 10 consecutive battles you have to beat.
So - it should unlock upon completing Arena mode, even if you never touched Campaign, if anyone is curious!