you cannot get this protection in regular run. If you never done any god-runs, i recommend you do so. I think you don't need to afraid any spoilers to this game, because in order to guess to, lets say, Sun-quest, you need to solve so hard fungus puzzles all around the world... Don't recommend you do so. Also, unlikely you would guess to some mechanic abuses, so...
Someone of pro-players could have seen a huge timer of polymorph protection on FuryForged (or any others) streams and don't understand how he get this. He did not made a video on this theme, and even me didn't find any video with this method... But, this is how to get a full polimorph protection.

The best polimorphine protection is eating a corpse of polymorph mage, it gives a protection from all polymorph types, exactly: polymophine, chaotic polymorphine, unstable polymorphine, any polymorph projectiles, and "Muodonmuutos"[noita.wiki.gg] spell (turn everyone into a sheep).

One corpse of poly-mage grants about 180 seconds of poly-protection.
You can find Polymorph mages[noita.wiki.gg] in Temple of the art (rarely, usually 1-3 on all the floor), in fungal caves (very rarely), and in wizard's den (a lot).
They are what we are going to use in our future polymorph farm. The second thing we need is bot necromancer[noita.wiki.gg], which revives nearby mobs if they are within a radius of about the third of the screen from it.

Meanwhile, killed enemies drops a corpse! with this mechanic we will make a poly-farm.

Best of all the places to make a farm is in the UDR (ultra dence rock) to the left from power plant, near the room of water.

Carefully dig from the power-plant and make a small room for the farm. We will lead necro-bot through this tunnel. Its better to not use super-fast black holes, so you dont break anything important. I used luminous drill with matter eater. Also, to make 2 separate rooms we need "long-distance cast" spell to dig through the ground and leave some walls so robot and poly-mage cant escape. In one of the rooms must be necro-bot, in another room - polymorph mage.

Necro-bot can be found in power plant, lead him to his room with homebringer teleport bolt[noita.wiki.gg], firstly teleport yourself in his room with long distance cast+small teleport bolt, then make a small gap and shoot through it with homebringer teleport bolt so robot teleports to you. Or you don't have to do a small gap to shoot at him, instead use drilling shot modifier to shoot through the wall. If you tried to teleport him by using LDC (Long Distance Cast) then he teleports to the place where LDC ends instead of to you. You can also use swapper spell.
Now dig down, to the wizard's den (the most dangerous location in the game) and lead poly-mage in his second room using the same method. Its better to get 4 "more love" perks so all enemies and poly-mages don't attack you, or go there already with some polymorph protection.

Now, when we build everything, we can start. Teleport to the room with poly-mage and start killing it. He drops his corpse and we eat it (you also need "iron stomach" perk so you dont explode), then poly-mage respawns and we killing it again, and so on. Also... You can't use consentrated light for killind the mage, because by using it he didnt leave any corpses...
I got 30000 seconds of poly-protection in 20 minutes, which is around 8 hours, pretty effective.

• Necromancer robot applies reviving effect not by instant, so you need to wait some time before killing it, and don't be greedy. Or else you will need to lead another mage to the farm.
• Green particles around enemy means that reviving effect by robot applied on him.
• You can make the same farm using extremely rare leggy mimic to get a ton of leggy perks, which you can reroll by using reroll machine from holy mountain (see my guide|not translated|). But it will leave a ton of mimic corpses (which you can delete using inf lifetime[noita.wiki.gg] black hole)
• There is another simmilar robot[noita.wiki.gg] from power plant which turns dead enemies into purple creatures, that looks simmilar like kolmisilma's minions.
• That one mage[noita.wiki.gg] didn't turns off this polymorph protection.
• There is a frame, when game checks your stains, which happens every 7200 frames (120 sec). In this frame you are vulnerable to the poly, despite having immunity effect. So, better not swimming in poly pools too much.

I hope this guide will help you in your runs. If you still have any questions you can ask them!
And don't forget to rate this guide! :)
If I write anything wrong - tell me, I'l try to fix this. (I'm not that good at english)
[somehow russian version of this guide got a lot more rewards than that]