Here is the full list of all 15 Windmills achievements.
You clashed swords with the skeleton king and emerged victorious
You banished the Demon of Conspiracy
You most certainly didn't fight a Pigman
You retrieved the crown of King Gurmathen
You bested the Necromancer in Glutton's Field
You defeated the Summoner at the Graves of Giants
You beat a hunter of Capes
You discovered and defeated the last surviving Dragon
You killed the beast that terrorized the three brothers at Prezeton
You hunted the witch with the funniest name
You made friends with a Minotaur, a creature which most certainly doesn't exist
You defeated the golem defender before the vanguard
You received the Flame of Rage
You defeated a Reaper and escaped your fate
You hunted the last witch