After breaking 8 Royal Seals, head back to Uhra and proceed to the Castle. Take the elevator there to find the last Royal Seal. Remember that you can't break it if you haven't broken the other 8 seals. Break it and the Golden Knight will appear.

Tolten will be the only one in this battle. For the first few turns, you can only "Accept" the challenges. And a few more turns, you will have full control of Tolten but I suggest healing yourself until the Knight mentions something about "protecting oneself". By that time, make sure to use Tolten's Complete Defense skill. The Knight will use the Ultimate Hit skill that will surely kill Tolten without Complete Defense active.

After enduring the attack, Tolten will learn the skill for himself. Use it against the Knight to end the battle.

After winning the battle, you'll get the Age of the King sword and King Ring. You'll unlock the corresponding achievement as well.