I finished my second nightmare run on the 3rd a few months so I think I can provide you with some inputs.
Party members: Kevin, Richard, Tita and Scherazard. Support member: Agate.
All members should be equipped with a +50% speed, MOV-2 quartz and the +15 speed, +2 MOV boots.
General strategy: - Kevin: equip him with the two auto CP regeneration accessories, which will restore 8 CP each turn. He will act as the support caster so the Godspeed quartz, which grealy cuts down the casting time of arts, is a must-have for him. As for magic, the following arts are essential: Zodiac (+25% STR and DEF), Clock Up Ex and Earth Wall. With this setup he can constantly spam Grail Sphere and provide all the necessary buffs for the team. It may sound strange but Healing isn't even needed because if done right, Cassius and Lowe won't have a chance to hit you at all. - Richard: our primary damage dealer. Be sure to have him armed with the ultimate weapon gotten from the Abyss as well as that +20% STR quartz. As for accessories, give him one which automatically recovers 5 CP each turn and any of the emblems that provides the most damage and decent speed. Personally, if my memory serves right, I used Sword Emperor Emblem. His strength at this point should be around 3400, and speed at around 140 - 150, it's been a while so I don't remember correctly. And also, give him the quartz that provides 20% critical for maximum damage output. His role is simple, just keep spamming his 25-CP craft like no tomorrow, and move back to the party when the cooldown starts to get long or when Schera's about to take her turn. BTW, Richard having the ability to cast MOV+2 and Clock Up Ex to share the burden with Kevin is recommended. - Tita: the best character to use against Cassius, as well as other human bosses. Without her, it would be almost impossible to beat Cassius the first time we encounter him without getting knocked out. Her role is simple, use the Orbal Gear and keep pushing the two guys away, wasting their next turn trying to running towards our party. A MOV+3 quartz is a must for Tita because move range is very important. As for accessories, give her something that boosts her low speed and her strength as well as the ultimate weapon from the Abyss. Her strength at this point should be at around 3200 on the status screen. - Scharazard: Equip her with whatever's left that is useful. She's here to spam Heaven's Kiss craft to remove all the cooldown from other party members so there's not much to put here. Be noted that she should have the worst speed among the four since if her gets the move before any of our other members, then her Heaven's Kiss is wasted on that member.
Be sure to put everyone to the back, and Cassius's first turn won't hit you. However, be sure to leave room for Tita to transform and don't have the party members too far away from the person standing at the center or else Earth Wall and MOV+2 will not cover the whole party.
As soon as the battle starts, follow this pattern: Kevin casts Zodiac, Tita transforms, Richard casts MOV+2 and Schera uses Heaven's Kiss. Then after Zodiac, immediately S-break with Kevin's Grail Sphere. Use Tita to push away whoever dared to run towards your party. A single hit from Richard will deal around 4000 - 5000 damage (~7000 when critical), and Tita will deal around 7000 - 9000 damage as well as the ability to knock back. She's not to be underestimated ;)
Just have Tita push the two away to have time for Kevin and Richard to cast Clock Up Ex on everyone and the setup's done. The rest of the fight is just a rampage of Richard and Tita. Earth Wall whenever necessary.
I finished this fight with the above setup and my party members didn't get hit even once. I really enjoyed this fight so I did several retries with the above setup, same results, got no hit at all.
Trying to maintain Max Guard does not work in this fight, Vegh. In fact, ir's counterproductive in most battles because you can buff your party, debuff the enemy (if possible) and finish battles in much less time than if you have to spend most of your rounds spamming Earth Guard/Wall.