The Praetor Slayer achievement normally requires the player to successfully beat the challenge "Say Your Praetors" in five different ways -- each one requiring you to kill a different Praetor. However, thanks to this major glitch with the way the challenge works, you can easily unlock this achievement once you've discovered a method to kill any of the five. First, discover or look up any method of successfully completing the challenge. Now, go back into the challenge, but this time buff up your Baneslayer Angel any way you like so that she can kill one of the other Praetors on the defense. Once the Praetor has been killed, bring up the menu and select Restart Challenge to begin again. This time, pick another Praetor and kill it as before, then restart again. Repeat the above process until you've killed each color of Praetor once, then finish by restarting and completing the challenge successfully. When the win screen appears, you'll be informed that you've completed the achievement! If not in quite the way the developers intended.