How to unlock the Defense Success Achievement.
Explanation and Preparation
As the name of the achievement would suggest, you have to successfully defend the ritual at the end of turn six. What the achievement does not tell you is that in order to defend the ritual someone actually has to attempt to sabotage it.
The way that I've found to get this achievement is quite simple: Start a game with two characters, guide one to the Infiltrator role and the other will be the Defender. At the end of round six they select their respective roles and you get the achievement. For this to work best you'll need to know the requirements for successfully performing the Infiltrator role and I'm not going to spoil that. You'll get enough elder signs to unlock the requirements while you get the other achievements if you really can't figure it out.
I am not sure if there are specific requirements to be able to successfully defend the ritual but the character I used had the most points in Body. All other stats were 4 or less if I remember correctly. My Defender was also "Player One" though I doubt that matters. It may be that the only requirement is for someone else to attempt to infiltrate the cult. Any feedback on this is welcome.
Step By Step
The main thing you're going to want to focus on is maxing the necessary stats for the character that will be the infiltrator. I typically focus on Madame Fufu's each turn with the Infiltrator because it can give you both necessary stats in the right proportion.
The character that will be Defender, as far as I know, can just be reasonably well rounded. The game does not suggest any stat requirements to defend and the only requirement I've found is that someone else must successfully infiltrate. So just have fun with this character.
Once you get to the end of cycle six and it's time for the ritual have your specialized character choose the FBI Infiltrator role and then have the other one choose the Defender role. Let the story play out and you should get the achievement.