Keep in mind that this guide is in constent update while following the news and finds in the discord. Some informations might be wrong or incomplete, I've noted those guides with "incomplete".
All the screenshots are from my own game, so every skin is unlockable, but we still don't know the exact conditions for some of them.

If you have more questions do not hesitate to pass by:

• Passtech Games Discord[discord.gg]
  
• Hidden skin discussion thread[discord.com]

Talent required :

• STARTING : Shapeshifter

Talent that helps :

• Short Wick (no more bomb stun)
  
• Explosive Rush (special on power)
  
• Adrenaline (to heal as human form)

If you take Shapeshifter and don't use it for the whole game, you'll be able to stay in human form.
Basically try to build around special, and have few charges / CD reduction on power so you can heal yourself as you don't have the werewolf

Your best bet is to go "Stimulant Vibes" as it's by far the most important damage booster for rats
Try to get a second charge of Special and/or special CD reduction to help you maintain rats alive for the longest possible time. You should be able to get the skin pretty easily this way.
Discord thread has been on the subject for a long time now, but it's still unclear how to unlock the skin.
Few people stated they obtained the skin without stimulating vibes. Sorry for the lack of information on this one


Talents that helps (?) :

• STARTING : Explosive Rats
  
• Stimulant Vibes
  
• Horde (more rats)
  
• Giant Rats
  
• Leeching Charm
  
• ULTIMATE : Plague of Rats + Rat King (level 10)

• mauvais_oeil
  
• Ugly Mirage (in comment section)

Needs more testing on this one, but it doesn't seem very hard to unlock.
Based on the skin name it might have something to do with the explosion from the trait "Explosive fire" but it's still unclear what to do excatly.





Talents required (?) :

• STARTING : Explosive Fire

Talents that helps:

• Furnace (could be mandatory for the +2s on ignite time)
  
• Breath of Fire (helps a lot maintaining the ignite in AoE)
  
• Fire Wings (helps further having ignite in AoE)
  
• Blazing Runes
  
• Fiery Slash
  
• ULTIMATE : Dragon's Flight + Immolation (level 10)

Try to keep ignite on enemies at all time, aim for large groups with your trait (reinforced by Fire Wings) or use your ultimate
It is advise to do this one in a 4-man group as it'll add a lot of enemies to match the party size

Killing them isn't required






Talents that helps:

• ULTIMATE : Snowstorm (kinda mandatory to make it easy)
  
• Radiant Crown (second charge of trait just in case)

Try to group the most enemies possible, then use Snowstorm when you think there's 15 enemies, then use trait to shatter them.
Don't take too long so Snowstorm doesn't kill them.

Easiest targets are probably the ghouls inside the Chapter 1 Ghoul boss cave or the birds objective on Chapter 2 with 2~5 pheonix that will spawn a lot of birds
It is advise to do this one in a 4-man group as it'll add a lot of enemies to match the party size

• weavermicro

So you have to use 9 wish along the game
It's possible to use wishes inside the sandman starting area[i.imgur.com]
Dying after doing the objective will still award you the skin

Talents required :

• ULTIMATE : Dream Wish

Talents that helps:

• STARTING : Jinniya’s Gift (one free wish)
  
• ULTIMATE : Infinite Wishes (level 10)

Build whatever you want to survive.
Make sure to do A LOT of money on the way, Leprechaun's Coin is probably the easiest way to achieve this one. Hope Diamond will help a lot but at the risk of losing your money, it is not required especially if you're not confident in surviving every fights.

Probably the easiest one. You can simply rely on the stat on the end screen to know how much you did
Shield doesn't need to absorb damage, just applying it is enough.
Water Bubble block damage doesn't count toward the achievement (as it is neither heal or shield)

Talents that helps:

• STARTING : Spring Water (heal on special) or Sea Dance (heal on trait)
  
• Protective Flow (shield on wisp going through)
  
• Soothing Presence (passive heal in fight)
  
• Backwash (with Spring Water)
  
• Tidal Wave (with Spring Water)
  
• ULTIMATE : Water Bomb + Healing Blast (level 10)

Just heal as much as you can.
Higher difficulty will help as enemies do more damage.
If you're going Spring Water : You can stack Special charges and CD reduction as much as possible to ensure this will be easier (I usually do >40k healing with this build)
Mermaid Tears helps A LOT. 4-stack of Fairy Dust also helps if going for Protective Flow.
Holy Grail is an easy addition to your kit.
Queen of Hearts Card, Unicorn Horn can bring more healing if required.

• Pabu

Do not take Unstable Cores or Master Puppet

Talents required :

• ULTIMATE : Meca-Puppet
  
• Large Family (+1 max dummy and trait CD reduction)

Talents that helps:

• STARTING: Dummyball or Twin Dummies
  
• Heart Ties (helps a lot keeping your dummies alive)
  
• Clockwork Medicine (also helps keeping your dummies alive)

Special and Defense additional charges and CD reductions helps a lot. Ultimate CD reduction helps also as you can try to build 2 Meca-Puppet with your ult without Nibelungen Ring if you got enough (4 Raven Eye is doable, 5 is better, 6 is comfortable) I don't recommend to rely on this thou
Build around vitality if you have Heart Ties.
You can build anything as long as it helps keeping your dummies alive.
The hardest part will be to secure the Nibelungen Ring to have the second ult charge.

You'll need to do it on Chapter 1, 2 and 3 boss respectively. Baba Yaga (Epilogue) isn't required as she doesn't exists in Adventure difficulty
Just being transformed while killing bosses doesn't seem to be enough. You'll have to take the last hit. Meaning doing it solo makes it easier unless you can communicate with your mates properly



Talents required :

• ULTIMATE : Transfiguration

Talents that helps:

• STARTING: Thirst for Immortality (vitality to survive while waiting for transformation, but becareful you'll have limited options to heal). Please note that you lose this vitality while transformed
  
• ULTIMATE : Perfect Copy (level 10)

As a general advice : play safe, let your team do the damage.
If you're solo try your best to time your ultimate with the boss death. It has a casting time before it activates so plan accordingly.
If you don't know all the characters in the game this one might be tricky so maybe keep it for later when you've played every characters and know their build.
The transfiguration ult is the strongest to me as it gives you basically a full optimized character with strong talents, but you need to react quickly and know how to survive with every characters.

Talents that helps:

• STARTING: Life Tap (blood for life price, help maintaining the winged form)
  
• Blood Reserve (more blood to spend)
  
• Puncture (+8 blood when dashing through enemies)

I advise strongly to do it the day so you permanently heal. Helps you survive.
Build around special charges and CD reduction so you can spam it, it will give you massive blood amount and it has comfortable intangible frames.
You can use timer until Master Nightmare (Chapter boss) to ensure you're doing it for the required duration.

Talents that helps:
To be defined

To be defined

Talents that helps:
To be defined

To be defined

• mauvais_oeil
  
• Pabu
  
• weavermicro
  
• other members of the community that did not wish to share their names 💖

My guide for achievement unlocks :
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3344256577