Here is the full list of all 14 Knightmares achievements.
Defeat the minotaur
Defeat the Clown
Defeat the vampire
Defeay the queen
Obtain I'm Back, one of the five ultimate fusions
Obtain Statikk, one of the five ultimate fusions
Obtain Void Pouch, one of the five ultimate fusions
Obtain Golden Armor, one of the five ultimate fusions
Obtain Raining Swords, one of the five ultimate fusions
Defeat all the bosses
Complete the game with a higher difficulty
Deal 100 damage to an enemy in one turn
Have a total of 40 armor during a combat
Fusion a card for the first time