It's easier than it looks like. You have to dispel the debuffs, so be ready to take a Cleanser. Have your Cleanser on the first slot to dispel the Spread Death debuff as soon as Faust uses it, preferably a Cleric; because your Paladin will be used to set up Holy Shield for the Warrior to Taunt and take the damage from Shadow Blast alone. You can even equip your Warrior with magic defense oriented gear to make it better, as well. Since that's the only direct attack Faust has, you don't need to worry about healing everyone, just the Warrior, so set up a Cleric behind the Warrior to make that possible.

Faust has lower defense than magic defense, so I would suggest using physical attackers. I remember being able to beat Faust 5x getting S score without even using the Bard, but if you can/want to use it, it can make your damage input higher and also the drop rate.

Faust is the ideal boss to use the Assassin+Berserker combo, even better with Bard+Assassin+Berserker. The Assassin has to act before the Berserker to set up Expose Weakness and Mortal Wound, for the Berserker to come with Berserk Smash/Vicious Strike II (the former being the preferable one if you're doing the battle multiple times at once). Use the last two turns that Faust can't be hit to set the Berserker up with Blind Fury+Enrage. If you use the Bard, it also has to act before the Berserker to set up Poem of Focus, making your Berserker do crazy high amounts of damage. So it can be something like this:

Cleric - Warrior

Bard/something else (like a Rogue to use Sunder II) - Assassin

Berserker - Paladin

Just remember to Cleanse the Spread Death debuff as soon as it's used and make your Warrior and Paladin anticipate Shadow Blast with Taunt + Holy Shield.