when you say keep anything from it, what exactly do you mean? I've been Enticing monsters into my party, have them pop out eggs until you get rare types, then just put them in a battle and farm for rare poach. Basically just have Ramza be whatever powerhouse class that will be able to one shot the mob (honestly often this is easy, I'll get some monsters that when they spawn from egg they'll have anywhere from 60 health to 250 health, currently). Eventually you'll have one that has low enough health to easily one shot without any trouble, just make Ramza a Ninja or any class with enough speed that he'll act first. Place the monster right in front of him, he kills it, you see if you get the rare carcass, if not, just restart battle from the start.
Takes very little time to chain attempts, I think I read somewhere the odds on rare carcass drops is 1/8 (though your RNG may vary, of course). Now if you mean by keeping stuff from restarting the battle, you can't just farm one mob over and over restarting the battle, but you can make sure you get the rare every time from the 1 mob by restarting until you get the good RNG.