How to beat Nephilim Weapon almost all her attacks can bypass Psycho armor
Copied from my reddit post explaining how to beat Nephilim:
I beat it on Paradox as soon as I saw it after getting my butt kicked the first time, heading back to the pocket castle to get a team together and coming back
What you need:
1. Est or another way to use hasted or preemptive sleep.
2. Your best dps characters, preferably ones that have attacks that scale off Agi (On Paradox difficulty Nephilim had 1.3 billion hp. Without good dps you'll be there for a very VERY long time and murphy's law might decide for you to miss your sleep spells and cause a run wipe.)
3. Galaxy Mage (Must know either Over slow or Over Haste, preferably both.)
4. A good buffer, preferably someone who can sing and dance and knows sea dweller dances
If you decide to fight it and get the brakes beat off of you, in the evaluation meeting Ilias will tell you its weak to sleep. That will be the crux of our strategy. So here's the plan, use your hasted sleep user or Est with preemptive Nightmare Smile to make Nephilim fall asleep. Reset till you succeed. Thats the easy part.
Now for the harder part, doing dps while keeping Nephilim immobilized.
Est/Your sleep user has 1 job and one job only: keep casting their sleep skill nonstop. Est works best since she gets double actions but any sleep user will do the job. If you have any ability or equipment that boosts status effect infliction chance, now is the time to bust it out because you do NOT want Nephilim Weapon taking a turn, everything she does except her dispel rotation will eliminate everyone with prejudice.
If the turn starts and she is asleep, even if your attacks wake her up she will lose the current turn. This is important to know and the key to effectively stunlocking her. Your time mage will be very busy: the first priority is sticking Nephilim with Over Slow, never let this drop off. This will give you time to resleep the boss next turn once she wakes up, without it she will outspeed you and kill everyone. Even if you have a hasted sleep user you still wont be safe because if they fail to sleep the boss thats the end of your run. Bottom line, you want Nephilim acting last all the time and Over Slow makes that happen.
Next on your time mages duties, Over Haste on your DPS. Its a stat up effect that stacks with All stat up buffs, and lets your dps go before your sleep users ideally so they can resleep the boss the same turn you wake them up with your dps, giving you 2 turns to sleep them instead of one.
When they arnt doing these 2 things, your time mage should be spamming stop on the boss, dont count on it hitting that often but if it does thats a bonus.
Your buffer should be throwing everything you got to boost damage. Weakening dance doesnt work so dont bother, however Dragon palace dance (75% weakness to all elements) and heroes song are a good start. If your Buffer happens to be a doll, Daisy Bell will boost your damage a ton as well. If Alchemy is available, using Dark Sword or Holy Sword to let your dps put corruption or Stigmata on the boss will boost your dps a nice amount, especially with conditionals based on those status effects. If you also want to pop a Homunculus Roar for double actions to speed things along, even better. Get creative with your buff suite and make sure they all stack, in part 3 things dont stack the same way they used to (For example regular stat boosts like energetic dance and all stat boosts like heros song dont stack.)
Your dps should be pumping out damage. Every stat charge skill (Full Power, Sophisticate, ect) has been nerfed heavily in part 3 so most of the time its better to just pump out dps constantly. I personally used Alice with the new Noble job, getting a nice Agi boost from buffs and Over Haste let her do great damage.
Keep balancing sleeping the boss and doing dps and eventually they will fall
The reward for your effort is an absolutely insane unit that has cracked equips for that point in the game. The battle itself gives a really shiny diamond and an accessory that turns normal attacks into chaotic assaults that hit 8 times, so definitely worth the effort to do the fight early if you want sick loot.
If you have any questions, ask me and ill elaborate