Here is the full list of all 20 Doug's Nightmare achievements.
Defeat Cthuda
Defeat Al Mushroone
Defeat Bacon Trio
Defeat Zeeble
Defeat Timegami
Defeat Psyrus
Complete the game
Complete a minigame
Pop all the balloons in a minigame
Hit 5 enemies with a single attack with a melee weapon
Hit 5 enemies with a single piercing shot
Defeat Anx
Get the first skin
Acquire 10 skins
Acquire 20 skins
Acquire all 33 skins
Get the first weapon
Acquire 5 weapons
Acquire 10 weapons
Acquire all 15 weapons