Hands down the hardest achievement in the game, you must defeat the Ethereal Queen on Infinity Mode.
Bare with me, this is all from memory :)
Preparation is the key for this battle so make sure you have the best equipment possible, have 3 fighters and 1 magic user. Here was my setup:Capell
Lvl:179 HP: 50k+ MP:6k+
Azureal Blade (see Haephestus Hammer solution for details)
Hydra Armor (anti-magic, very important, forged by Edward)
Eclipse Helmet (anti-magic, forged by Edward)
Kylin Greaves(Hp Regeneration, forged by Edward)
**Casandra Ring (Hp+10%, MP+10%, Quest Item!)
Sacrificial Doll (revives character one time when they die)Edward
Lvl.176 HP: 55k+ MP: 5K+
Karathos Blade(forged by Edward)
Karathos Armor(Dropped by Seraphic Gate Dragonbone Shrine Boss)
Eclipse Helmet (anti-magic, forged by Edward)
Eclipse Greaves(forged by Edward)
Ring of Life(forged by Kristofer)
Lvl: 176 HP: 30k+ MP: 8k+
Cadaceous ( forged by Eugene)
Top Crafted Armor from Eugene
Top Crafted Boots from Kristofer
Bloody Hat (HP Regeneration, forged by Kristofer)
Magic Aria (casting time down, forged by Kristofer)
Lvl: 176 HP: 35k+ MP: 5k+
Nighthawk(chest after defeating Gabriel)
Phoenix Feather equipment (found in different chests)
Eternal Rose (HP Regeneration, forged by Kristofer, but you will need access to Sigmund to forge Lei and Blossom ring required for this)
Besides having top crafted weapons/armor for your chosen characters make sure you have: 99 Miraculous Medicines, 99 of every berry you can buy, 99 Sacrificial Dolls.
Now for the battle, before you even get to the Queen, you will have to fight Leonid again. Use the same tactics as before and don't try to be to hasty, you will want to use the least amount of Miraculous Medicines and Sacrificial Dolls as possible (you can even wait to equip S.D's until you fight the Queen).
Now once Leonid is out of the way you will fight the Queen, first off I will go over her attacks:
Magic Spells: She will use various magic spells, if you're fighters have either hydra armor or eclipse helmet on they will not be harmed by this, just be careful of your other characters. (Also this will be noted by a blue circle around her)
Seraphic Law: Very common and very dangerous attack, pretty much a homing fireball attack (non-magic attack) no way to avoid it unless very far away, will most likely kill a character if all fireballs hit.
Southern Cross: She won't start using this attack until at about half HP I believe. A flurry of physical attacks (similar to Gabriels Noble Play) easily avoided and escaped from.
Accused: DO NOT GET CAUGHT BY THIS ATTACK! Once she starts using Southern Cross, she will start using this attack as well. She will mumble something, then slam her staff into the ground and cause any characters nearby to be stunned and dealt 1 damage, she will then release a million bubbles that will do massive damage and WILL kill anyone caught in it.
Now for the fight: As Capell, do not fight her, you will be responsible for re-equipping S.D's when people die and killing foespheres (kill as many as you can, but don't put yourself in danger to get to them), nothing else, let your other characters deal the damage to her, and spam that "Y" button a lot to keep your characters as healed as possible.
Keep as far a distance from her as you can, but not to far so that your melee fighters won't fight her. Watch the battle and when you see Survived! pop up next to your ally (and they have 1HP) that means they have used their Doll, go into your menu and equip another on them.
It's a simple procedure, but you must stay on top of things or you can go down quickly.
This fight will take a VERY long time, expect around a 1 hour battle.
*Note: If you are having trouble with this fight then consider using the "Grimoire Method" to increase your HP into the hundreds of thousands.
I warn you though that doing this method takes MANY
hours (you'll see if you start farming) and honestly is a waste of time considering you could level grind to Level 200 much quicker and be able to take her down with much less hassle than I had at 179.
I ended my Infinity Run at 64-65 hours.
Much thanks to Van Uden
for many suggestions and improvements as well as the walkthrough to get the Casandra Ring
**After the incident at Halgita go to the church (1F right in front of the entrance). Connect with Eugene and take the elevator up to 3F. Step out of the elevator and take a left. You'll walk past a room with 2 guards. After that take a left and you'll find Karin the Aristo walking around. Talk to her and she'll mention the Queen lost her ring. She suggests to look in her room and maybe ask some soldiers.
After the conversation walk back to the elevator and take a left. You'll run into a door with 2 guards. This is the Queen's room. Go into the room and immediately exit. Talk to the guards and one of them will mention that they noticed animals running about when they were cleaning up.
Disconnect with Eugene and take the elevator back to 1F. From the elevator go straight ahead into the shopping street. At the end of the street you'll find Rico. Connect with him and go back to the elevator. Go to 2F and take a left after you exit the elevator. Near the end of the hallway make a left and enter the last room on your left. There is a rat running around there, talk to him and he will return the ring to you.
Disconnect with Rico and go back to the church on 1F to connect to Eugene again. Take the elevator to 3F and go talk to Karin the Aristo again. After talking to her walk back to the elevator. Before reaching the elevator you walk past a room with 2 guards on your right. This is the room Queen Svala is in (not the same room you had to walk in before talking to the guard at the start of the quest). Talk to the Queen and you will hand over her ring and in return finally receive your reward, the Casandra Ring.