Death's part of gameplay, characters should die in this game. There's an achievement locked behind 'character death'. Game autosaves after every successful travel. If the character's poisoned early and you won't cure them, might wanna reload to before the poisoning happened. All characters on all game modes own a room at the orphanage. A golden whistle icon on the map of a region denotes mission objective on some missions, character presence at region may be required. The harvesting skills double the amount received. Holding the jump button makes character jump higher. Crouching is a way to adjust height to survive certain attacks and hazards such as laser beams. You can overall save stamina by leaping before it's consumed and it's possible to stand atop some destructible wall stones. DEEP IN ABYSS tip: Figure out which enemy's weak to which poisoned arrow as one shot might be lethal for them.

If you're ever unsure of what to do, please check all quest logs to see if there's anything active there. If there isn't, then your current objective's to either find a path to the deeper layer or to survive the ascension back to town, maybe both. Remember to use autosaves for scouting and test things because dying over and over's expected.

Might wanna look into removing timed spawns at: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2860059436
Beware an early main quest on the first layer of DEEP IN ABYSS relies on the spawns being active. If you play fast/use smoke signal thingies, you won't ever need it.

Please comment which achievement you're trying to get so the guide'll be updated with more information and you'll be added to the list of contributors. Listed below are the achievements as they appear on steam.

Obtain all achievements.

Clear HELLO ABYSS.

Character has some traits in this game mode: Static character level. Unlimited durability. Many recipes unlocked. Random spawn timer disabled. No Item Box.

Make sure to carry Salt. You can only get Salt in town by getting it from the food merchant. Some people have unironically said that 99 is too little, some people unironically say that 1 is too many. If you run out of food, you can progress on an empty stomach as long as the routes don't require fullness/stamina. The advice for this game mode is to go fast. You won't miss any achievements in this game mode (HELLO ABYSS) by going fast, unlike the other game mode (DEEP IN ABYSS).
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2995604236

Become a Red Whistle.

Lunge skill recommended.

Become a Blue Whistle.

Main quest. Progress-based Fast Travel's unlocked. Might wanna cook Crumbled Boiled Egg. Movement speed skill recommended.

Become a Moon Whistle.

Main quest. Regular climbing stamina reduction and regular climbing speed skills recommended.

Become a Black Whistle.

Main quest.

Become a White Whistle.

Main quest. Travel costs are greatly reduced.

Clear DEEP IN ABYSS.

Clear Riko's episode.

There seems to be a problem awarding some steam achievements. In order to avoid the problem, get this achievement before entering 5th Layer or before defeating Tiare. This achievement can only be acquired in 'DEEP IN ABYSS' game mode. The path to 4th Layer's blocked by main quest, so be sure to progress main quest.

Clear Reg's episode.

The location's the same as Riko's episode, but it only changes after defeating Tiare. Some players have been unable to get the achievement because they've defeated Tiare before completing Riko's episode on their only DEEP IN ABYSS character. Please complete Riko's episode as soon as you comfortably can (completing Riko's episode isn't very straightforward to many players, as it can start and lock travel after a boss fight; there's a fairly challenging/unknown route to brave, resources, etc) in order to unlock everything.

Defeat Detchuanga.

Can crouch under their wide spinning blades attack. They're immune to poison. Good luck.

Receive Nanachi's best dish review.

Die on Goblets of Giants through fall damage to unlock Nanachi and there eventually will be a quest to give them food. It may come after a material gathering quest or more, but when it does show up as a quest (may not work if you do it when it's not a quest that's been registered on the quest log), complete it with either:
Corpse Weeper Steak;
Demonfish Miso Soup;
Fried Bird with Skin;
Fried Meat Platter;
Gorgeous Hot Pot;
Hamashirama Pot;
Hamashirama Sashimi;
Hearthy Stew;
Laffi's Special Lunch;
Memorable Stew;
Pan-Fried Demonfish;
Rare Fish Hot Pot;
Rare Fish Sashimi;
Salted Rare Fish Roast.

Reach the first layer the first time.

On DEEP IN ABYSS game mode, may harvest fruit to keep in your Box at your Orphanage Room to later bring down with you as heavy snacks to open room for other things such as relics. When on Multilayer Hill, before the promotion trial, it's recommended to enter Forest of Temptation so the character memorizes the route for a future fast travel option even though, according to cave raider guild rules, red whistles aren't allowed to dive that far down.

Reach the second layer the first time.

For DEEP IN ABYSS:
Bring ropes. Stagger the second layer's interference unit by swinging a melee attack at their flank or firing a ranged attack at their chest. Don't let them burrow. After defeating the boss, travel to the next layer to pre-register it for the next fast travel upgrade if you can spare the resources. Crumbled Boiled Egg for hunger and health, poisoned red arrow for potential foreign ambush. Maybe purchase the arrows to save skill point(s). Bosses are immune to poison. Some say they can be paralyzed by certain effects.

Reach the third layer the first time.

If you push ice and a big, flaming boulder clears the path forward, it'll be sealed on the way back; so don't count on it remaining clear for you to go back the way you came from. Climb the wall up to the very top of the Crimson Splitjaw room at Glass Layer to a place there's floor to stand on and blow your whistle at the center to make a path. Dupe/Bamboozle/Trick the Madokajacks (through character movement) into missing their dive attack on purpose so you have enough time to get into the next tunnels.

Ascension:
Climbing without fast travel's required after defeating the third layers' interference unit (main quest has character go into the next layer for a bit and there's a poop-eating child near the entrance, take care starting their quest if you're very low on resources, but do it -Riko's episode- before progressing main quest at Ido Front in the fifth layer. Might wanna ignore them on the first visit, but do it as soon as possible after acquiring black whistle fast travel. You decide.). The path back to town's described here along with advice: https://steamhost.cn/steamcommunity_com/app/1324340/discussions/0/3728450372147621173

Reach the fourth layer the first time.

There's a waterslide to the left that might save you rope(s?). Stay away from Orb Piercers as they won't attack what's outside their territory. Try visiting the next layer before or after defeating the layer's boss so the destination becomes a valid option for future fast travel.

Sidenote for black whistles:
The old man delivery quest can be completed without delivering the relic if you equip it.

Reach the fifth layer the first time.

Bring enough weapon durability to clear two different boss fights while hauling the relics asked by The Lord of Dawn, White Whistle Bondrewd. The last hit depletes a lot of durability, so you wanna save low durability weapon to deal that final, weapon-breaking, hit.

Visit over 40 stages.

Cook a dish for the first time.

Craft something for the first time.

Collected Information with the Monocular for the first time.

Obtained Relic for the first time.

Collected 10 Relics.

Collected 50 Relics.

Collected 100 Relics.

Collected 200 Relics.

Learn a new skill for the first time.

Unlocked 10 skills.

Unlocked 30 skills.

Unlocked 50 skills.

Be affected by ascension strains.

Suffer the 1st to 5th layer's strains.

Do it on the same dive. Much easier after defeating Tiare.

This guide would never have come to fruition without the help and inspiration from:
Virgil KoMETA
https://steamhost.cn/steamcommunity_com/id/HomuraCrimsonSquad
https://steamhost.cn/steamcommunity_com/id/systemrush
https://steamhost.cn/steamcommunity_com/profiles/76561198168333062
https://steamhost.cn/steamcommunity_com/profiles/76561198448827329
https://steamhost.cn/steamcommunity_com/id/Mister-I
https://steamhost.cn/steamcommunity_com/id/SorajimaAnzu
https://steamhost.cn/steamcommunity_com/id/Reaper1441
https://steamhost.cn/steamcommunity_com/id/M07H34
If you're not on the steam software and you wanna open any link through it, add
steam://openurl/
before the link. Feel free to try on my profile for reference:
https://steamhost.cn/steamcommunity_com/id/Eoun5118
steam://openurl/https://steamhost.cn/steamcommunity_com/id/Eoun5118