Here is the full list of all 23 Monster Slayers trophies - there are 3 bronze, 13 silver, 6 gold and 1 platinum.
Defeat the Harbinger with a Rogue.
Reach level 15 with a hero.
Have a hero with at least 200 HP.
Defeat a Boss.
Complete the Tutorial.
Collect all trophies to unlock the platinum
Defeat the Harbinger with a Ranger.
Defeat the Harbinger with a Wizard.
Defeat the Harbinger with a Knight.
Defeat the Harbinger with a Barbarian.
Defeat the Harbinger with a Cleric.
Defeat the Harbinger with all 6 starting classes.
Defeat the Harbinger with an Assassin.
Defeat the Harbinger with a Necromancer.
Defeat the Harbinger with a Beastmaster.
Defeat the Harbinger with an Apothecary.
Defeat the Harbinger with a Monk.
Defeat the Harbinger with a Brute.
Complete a game in Legendary Mode.
Defeat the Harbinger with a Merchant.
Defeat the Harbinger with a Dragon.
Complete Legendary mode with 6 different classes.
Complete Legendary mode with 12 different classes.