If a Tigress goes with you to the final battle as a guide, then you can get them from her. You can also find them in the deep forest (the place where the goblins stand) and save them. The Purr skill will also help in battle.
You don't have to kill the rats. You can't die in battle with your fellow tribesmen, but you can gain experience. That is, go into battle with one hp and use rations later.
