This battle is against Clan Ox. The clan leader asks Marche if he knows about the person that the palace has a bounty on. One of the clan members says that Marche looks a lot like the guy on the wanted poster. Marche doesn't hear that comment, so when the clan leader asks for his name, he answers truthfully, and the clan attacks, realizing that Marche is the guy who has a bounty on his head.
The Paladin, Fighter, and Black Mage have attacks that will do a lot of damage, so you should focus on getting rid of them first. The turn order is important to monitor, because you will really want to have as many of your units attacking one right after the other as you can, to make sure you can take down those three, with their high HP count. If you have a way to affect the turn order to your advantage, do it. Putting one of those three to sleep is a good idea.
You aren't out of the woods after getting rid of those three, but the other enemies are a lot less likely to KO you quickly. The Sage can poison your group with an area attack, so don't stand close to each other or you'll have several people poisoned. It would be wise to bring a strong healer.