Here is the full list of all 26 Felix The Reaper achievements.
You Completed All Main Campaign Levels in Chapter One
You Completed the Tutorial Stage
You Completed a Level Without Getting Caught in the Sun
You Completed All Main Campaign Levels in Chapter Two
You Completed All Main Campaign Levels in Chapter Three
You Completed All Main Campaign Levels in Chapter Four
You Completed All Main Campaign Levels in Chapter Five
You Completed Your First Hardcore Level
You Completed Your First Time Trial Level
You Collected All Coins in a Single Hardcore Level
You Completed All Hardcore Levels in Chapter One
You Completed All Hardcore Levels in Chapter Two
You Completed All Hardcore Levels in Chapter Three
You Completed All Hardcore Levels in Chapter Four
You Completed All Hardcore Levels in Chapter Five
You Completed a Standard Level in 16 Seconds or Less
You Completed a Hardcore Level in 40 Seconds or Less
You Completed All Main Campaign Levels
You Got Four Skulls in Each of 5 Levels
You Got Four Skulls in Each of 10 Levels
You Got Four Skulls in Each of 15 Levels
You Got Four Skulls in Each of 20 Levels
You Completed All the Time Trials
You Got ALL Skulls in Each of the Main Campaign Levels
You Found All Silver Coins in All the Hardcore Levels
You Collected All Trophies/Achievements Listed