After unlocking Moss Blanket, jump down the tree trunk and go right. Going up you will find another circle doors, requiring another key. Once opened, you will need a few plorts:

Tabby
Phospor
Rock
Honey
Boom
Rad

And place them in the corresponding pillar (they should look similar to slime they are coming from). Once you place them, last gate opens. Go through it and you get the achievement.