Here is the full list of all 13 Oh Darling! I Hate To Tell You This... achievements.
Beat the Holy Orb Boss
Beat Your Inner Demon
Beat the Mom Boss
Beat all the Big Bosses
Beat the Lady
Beat the Cyclops
Choose the Exploding Bullet
Choose the Double Bullet
Choose the Big Bullet
Choose all the Bullet types
Complete the sentence
Kill 120 Demons
Kill 666 Demons