From Scratchpad

Snakeman requires that you at least be level fifteen for this quest,but I recommend that you reach level twenty first. I also recommend that you either capture or fuse one level twenty devil that you do not mind sacrificing.(Honestly, it's easier to just go to nakano and capture a jack lantern, they're level 20 right off the bat)

To start this Act, talk to Snakeman while you have a Suginami Bronze plate in your inventory. If you do not have one, go buy one at the Suginami Dungeon for one hundred makka. Once you started the Act, go talk to the Jakyo Manor Master and he will ask that you get the story item called the Stone of the Old World(旧世界の石).

To get this story item, go to X=21\Y=23 of the Suginami map and talk to the miner there. He will explain to you that only a devil can get the stone from the ruins behind him. He recommends that you let a high-level devil handle the job. A devil that is too weak will only die. If you decide to send in a weak devil and it dies, you alignment will move slightly to Chaos, and the demon will be gone permanently. A devil that is at least level twenty is practically guaranteed to survive and bring back the stone with it.

With your level twenty devil summoned, talk to the miner again and have him send in your devil. You should get the story item. The item should be listed under the 貴重品 option in your ITEM menu.

Return to the Jakyo Manor in Third Home while you still have your disposable devil summoned. When you talk to the Master with both the Stone in hand a a devil that is at least level twenty summoned, he will take both stone and devil from you. He will not prompt you in any fashion, so be prepared before you talk to him. Now you must wait until the next Full Moon before you can continue this quest.

On the Full Moon, the Jakyo Master will hand you the Suginami Ash(スギナミ灰)plate, a special event plate for the Suginami Dungeon. If the plate disappears from your inventory before you finish this Act, you can return to the Master for another plate at any point in the game, without having to sacrifice a devil.

At six floors, Ash may seem like a long dungeon, but it is possible to break up the dungeon into two floor increments. If you noticed, there is a Jack Frost NPC in the corner of the starting room. He is looking for two story items, the Shining Blue Ore(青く輝く鉱石) and the Cold Ore(冷たい鉱石). If you show him one of the ores by selecting (Ore Name)を見せる, he will allow you use a shortcut that will transport you to the end teleporters of floors two and four. To get these stones, you must beat clear the final rooms of floor two and floor of enemies.These two rooms are filled with the Fallen Angels フォルネウス and デカラビア. The Blue Ore, which gives you access to the shortcut to floor two, will automatically be placed in your inventory when you finish off the フォルネウス on floor two. Killing the デカラビア on floor four will give you the ore that gives you access to the shortcut to floor four.

Floor Six is the boss room, an area that is populated by three powerful Fallen Angels. Even if you managed to solo the dungeon by yourself, you will fast more trouble if you attempt to try to solo the boss room. It is possible to do so, but it is much easier to have some party members. Each boss has different weaknesses, so plan accordingly.

アンドラス, the owl man, is weak against 魔力(Magic Force) and 貫通(Pierce).That means attacks like Magic Shot and regular rifle attacks will be most effective against him. アンドラス will reflect or nulify holy attacks(破魔),shotguns(散弾),handguns(射撃), or blowback(衝撃) attacks.

フォルネウス, the rayfish, has an average offense, but his defenses will make him a tough boss to kill. He is immune to shotguns(散弾), ice(氷結), Magic Force(魔力), and all magic. Even if you try physical attacks, you will not be doing much damage. This boss' only weak spot is fire elemental physical attacks and lightning elemental physical attacks. The only ones that have access to these types of attacks would be people with elemental machettes or devils with the Heatwave(ヒートウェーブ) or Lightning Thrust(稲妻突き) skills. If you have a Taaraka or any devil that inherited the above attacks during fusion, I recommend you use them. Otherwise, save this one boss for last.

デカラビア, the starfish, may be the toughest one. Not only can he heal himself and the others, but he can also use Zanma and Maha-Zanma. If you do not use the dodge skills fast enough, you and perhaps your allies as well will take at least ninety points of damage. Luckily, this boss has the fewest immunities; デカラビア will only reflect or nullify Thrust(突撃), Blowback(衝撃), and fire(火炎). He also have the most weaknesses. All blunt instruments(打撃), rifles(貫通), shotguns(散弾), and holy attacks (破魔) will work against him.

Once you finish the fight, return to Snakeman for a two thousand and five hundred makka reward. For an extra reward, team up with a person who started Ash and talk to the Jack Frost at the dungeon entrance. If you give him your ores, he will give you a Jack Frost Pendant. Do not worry, you will still have access to your shortcuts.