Here is the full list of all 33 Side Quest achievements.
Defeat 50 Zombies
Complete all optional challenges
Throw 20 enemies in the water at the Cemetery or the Village
Defeat Frank without getting hit
Defeat 200 Goblins
Get 25 hits during a combo
Finish the game without speeding up any scenes
Finish the game in under 40 minutes
Get to the Final Boss without getting hit
Finish the Lonely Knight
Get 50 hits during a combo
Get 100 hits during a combo
Get to the end of the Dungeon without getting hit
Get to the end of the Cemetery without getting hit
Get to the end of the Village without getting hit
Get to the end of the Castle's Entrance without getting hit
Get to the end of the Throne Hall without getting hit
Get to the end of the Library without getting hit
Get to the end of the Training Range without getting hit
Get to the end of the Treasure Room without getting hit
Get to the Top of the Castle without getting hit
Defeat the Lonely Knight without getting hit
Defeat the Last Boss without getting hit
Finish the game without completing any challenge
Defeat 150 Knights
Throw 20 enemies at the Dungeon's pit of from the top of the Castle
Lit all the inflamable hay stacks
Finish the game speeding up all scenes
Make an enemy kill another enemy
Make 5 enemies kill themselves
Kill all enemies in the main game without dying
Get to the Final Boss in under 30 minutes
Defeat 50 Baby Dragons