This fight is totally optional. Your quest initially was to find the ID number of the student that is engraved onto a sword just as you enter the tomb. If you choose to go on into the tomb to defeat the two monsters there you will obtain yourself a reasonably strong GF. I recommend defeating these bosses and acquiring this GF because it is relatively easy to do and since the Brothers attack is Earth based it has a fair effect on Earth weak monsters.
The first boss that you must fight is Sacred. Sacred is the little brother to another beast called Minotaur. By using the map to the tomb and going to the right most room you will find Sacred.
HP: 855- 36,375
AP: 20
Weak vs. Poison and Wind
Immune to Earth
inside the tomb.
The only other problem you will face is that Sacred has a pretty powerful basic attack. Fortunately he carries the spell Protect which will decrease the power of his attack greatly if casted on each of you party members. Just as Float, Protect magic wears off to so you might have to re-cast but Sacred carries this spell so you do not have to have a huge stock ready.
After defeating Sacred he will run away leaving you with nothing to gain from the fight. Fortunately Sacred is hiding somewhere else inside the tomb, and this time he has a friend.