Final Fantasy Tactics Walkthrough

You'll start off in a disadvantaged position, with the main route to the scene of the battle likely blocked by a high wall. The best thing you can do is to give as many people as possible Ignore Height so you can get to Reis faster. If you can't do this, start your best movers and fastest characters at the back of the formation so they can make more progress towards the front lines.

Reis will take out many of the enemies on her own but will eventually need backup, and the faster you can get Ramza or Orlandu over there, the better. She'll take a beating but she's got good HP, and the damage dealt to her is low due to her Brave and Faith stats. Holy's range will come in handy here, especially since the Archaic Demon is weak to it. That said, defending Reis is the objective of the battle, so don't leave her alone too long lest she become endangered. Focus on getting your units to places where they can keep Reis safe, including getting a healer into position if you can't end the battle quickly.

There are fun monsters here to try to invite if you feel like it, too.

Following the battle, it turns out that Reis is who Beowulf was looking for in the first place. You'll have the opportunity to recruit both of them permanently - do it. Even if you don't want them, further sidequests require them to be in your party. You'll also get the stone you'll need for that fancy machine you left in Goug, so that's your next stop in continuing your sidequests.