Here is the full list of all 17 Biomass achievements.
Befriend the Red Mother
Kill the Red Mother
Kill the Prisoner of War
Kill the Behemoth
Stop the Corruption in the Village
Beat the Corruption outside the Gate on first attempt
Kill the Duke's Right Hand in the East Tower
Suspend the Corruption
Finalise the Process
Kill the Twin Shells
Kill the Marionette
Kill the Duke's Left Hand
Kill the Angel
Kill Legion
Help the Red Mother against her foes
Defeat the Drifter outside the Meridian Gate
Gang up against the Red Mother