Here is the full list of all 32 Monstrous achievements.
Activate a Super for the first time.
Activate 100 Supers.
Defeat a rival monster for the first time.
Defeat 100 rival monsters.
Perform a Finisher for the first time.
Perform 100 Finishers.
Complete your first game session.
Win your first game victory.
Win 10 game victories.
Win a game as the Vampiress.
Win a game as the Ghoul.
Win a game as the Promethean.
Win a game as the Witch.
Kill 1000 Thugs.
Kill 1000 Cultists.
Kill 1000 Hunters.
Kill the Thug Ace.
Kill the Cultist Warlock.
Kill the Hunter Master.
Interact with all city NPCs.
Loot 100 items.
Loot your first item.
Collect 1000 coins.
Earn the Capitalist medal 10 times.
Earn the Sheriff medal 10 times.
Earn the Enforcer medal 10 times.
Earn the Assassin medal 10 times.
Start your lore collection.
Collect 10 lore bits.
Win a game on Hard mode.
Win 10 games on Hard mode.
Start your first game.