Here is the full list of all 37 Hordebreaker achievements.
Move at maximum move speed.
Kill 1000 Zombies
Kill 300 Harpies
Kill 250 Automata
Kill 750 Imps
Kill 500 Drow
Kill 500 Goblins
Kill 250 Dryads
Kill 50 Werewolves
Kill 250 Troggs
Kill 250 Daeva
Clear the Autumn Farmland stage
Clear the Snow Ruins stage
Clear the Black Palace stage
Defeat Kojisaki
Defeat Neverfound
Defeat Boklund
Defeat Kojisaki in Campaign without getting hit by any of her clones
Defeat Neverfound in Campaign using only fireworks and the Recursive Rocket
Defeat Boklund in Campaign without getting hit by any lightning attacks
Fully upgrade your health and speed
Fully upgrade the sword
Fully upgrade the bowgun
Fully upgrade the fireworks
Fully upgrade the Typhoon Blade
Fully upgrade the Dynami Bolt
Fully upgrade the Recursive Rocket
Have every upgrade at the same time
Get far enough in Endless that you start losing frames (or reach wave 150)
Read the completed journal
Eat 300 pieces of meat off the ground
Watch the "credits"
Take 40+ damage from your own Recursive Rocket and die
Hit a Werewolf with 8 or more simultaneous Recursive Rocket explosions
Deal at least 3000 damage with a single Typhoon Blade
Hit 50 enemies with a single Dynami Bolt
Have an Automaton kill multiple Imps with a single shot