First off you need to complete the Wickerman Challenge. I have linked a video it isn't my video and all credit for it goes to MrFreeze2244. He will show you how to get the scarecrow disguise climb the water tower and shoot 4 bells with the sniper rifle. Once that is done, the real fun begins. I ran around killing everyone, making sure not to kill the targets (save them for last). Once you have most of the enemies eliminated switch out of the scarecrow disguise pull out your pistol and kill the few remaining guards. Now you can do all the challenges you want just keep reloading your saves. I hope this helps!