Trade the testimony to the door, the page will rip and you'll be transported in, stripping any buffs you have on you except Signet/Sanction and food. (NOTE: The Testimony appears to be normal and will remain in your inventory, but it is like a BCNM orb in that once you use it to trigger this fight, you cannot use the same Testimony again. If you lose the fight against Raubahn, you must farm another Blue Mage's Testimony from Mamool Ja Mimickers.)
You have 10 minutes as with any of the 70 Limit Break quests.
He knows all level 70 and below spells, and will use any of them regardless of what you have set when you go in. He also casts any of the spells in about 1-2 seconds as opposed to the spell's normal casting time.
The primary strategy used for this fight is to deal out as much damage as you can before he kills you.
Starting with Frost Breath is a great way to begin the fight.
Magic Fruit or any other healing sources tend to be more of a hindrance than help in this fight.
Bringing a Yagudo Drink and a couple Ethers is a good idea as MP can quickly become an issue.
Unfortunately this fight tends to rely a great deal on which spells Raubahn chooses to cast. Too many damaging spells will kill you, but if he uses spells like Geist Wall, and even Magic Fruit it only helps you as it gives you that much more time to do more damage to him.
Raubahn has access to Savage Blade, hit for about 600.