Here is the full list of all 11 Dark Gnome achievements.
Defeat the jerky Rat King Charles
Defeat the mean Cat Boss Mr Fluff
Defeat the Fungus Boss so the mushrooms retreat from the ruins
Defeat the Frog Boss so that the goo will melt off the ruins
Defeat the troll
Defeat the Snake Boss in the desert
Defeat the bear in the oak wood
Defeat the Moth Boss to get the required items for your bait
Defeat the Frog Boss to free the dog
Defeat the Rat Boss to cross the sewers
Defeat the proconsul and his retinue in the dog palace