To unlock this achievement, you must complete all 27 Himmelstein challenges - 13 assassinations and 14 feats. All are possible in single-player, although some of them, especially the timed ones, could be easier with a partner. This guide is for doing them solo.
I recommend setting Performance/Scoring HUD On, Instinct On, Objectives Auto or Always On, NPC Icons On, Challenges to Full and Challenges HUD On. This guide is written assuming you are using these settings, so if you're going to do things your own way, that's fine, but don't blame me if the challenges are harder or this guide doesn't work for you.
If you're stuck on a particular challenge, move on to some others. The more XP you get, you will unlock more upgrades to your rifle. Faster reloads, larger magazines, faster fire rate, steadier aim, more aim slowdown time (squeeze 
lightly while aiming), etc. all make the difficult challenges easier.
If you complete your last challenge before you eliminate the three main targets, the achievement will unlock after you eliminate them.
All times in this guide refer to the "time remaining" indicator in the lower left (Objectives must be set to Auto or Always On). The timing can vary or be thrown off completely by your actions, so my times are always based on you doing nothing other than what I've specified for that particular challenge.
W = toward the left of your screen, E = toward the right of your screen
N = toward the background, S = toward the foreground
ASSASSINATIONS
1. A Head Off the Curve - eliminate all targets with headshots. Self-explanatory. You will need to be pretty stealthy with this (i.e. hide the bodies), because once people start evacuating, you aren't going to get headshots on them without tons of luck.
2. Sleeping With the Fishes - eliminate and hide five targets in water. Easy. Just shoot a target when it is standing on the near side (with respect to you) of a pool.
3. Clean Slate - eliminate and hide all targets. There are many ways to hide them, including in water, in bushes, over ledges, and through the windows and skylights on the upper floors. Having a large object fall on them by accident, such as the chandelier or the statue in the center of the labyrinth, also works. This is a difficult, but fun challenge where patience is rewarded.
4. Smokestack Lightnin' - eliminate three targets by electrocution within thirty seconds. Quite easy to do at the beginning. Shoot around the helicopter pad area to keep the bodyguards there. Shoot the water faucets. Wait for the helicopter to land. Shoot some of the sprinklers. Now you will have Lee and three bodyguards in this area where there is a lot of water on the ground. Shoot the lights and you should get at least three of them. Faucets, sprinklers, and lights all are highlighted with Instinct. (Credit to https://www.shacknews.com/article/105365/the-last-yardbird-c... for this tactic.)
5. Heavy Burden - eliminate the three main targets with the same suspended object. Wait until around 4:00. The three main targets will gather below the chandelier. Shoot the cord holding the chandelier. (Note for challenge #13 that this moment occurs during the bridal waltz.)
6. Killer Shot - eliminate Dorian Lang and Guillaume Maison with the loudspeaker outside the art gallery. Shoot the photographer's lamp and raise the flag in the labyrinth at the beginning of the mission. Lang and Maison will meet in the art gallery, and at about 3:30 they will come outside and pause under the speaker. (If you don't shoot the lamp and raise the flag they will still meet in the art gallery, but it will be earlier, and they will leave out the back of the room.)
7. Got a Light? - eliminate the three main targets in the same explosion. At the beginning of the mission, shoot the chandelier cord . At around 4:00, when the targets would ordinarily be standing under the chandelier, they will instead go to room on the 3rd floor of the west wing. There is a cannon on the west side of this room. Shoot the fuse.
8. Touch and Go - eliminate the three main targets while they are escaping. Kill a bodyguard in plain sight or do something else to cause a panic (without killing a main target). All three targets will soon begin evacuating. The easiest way to kill a running target is with shockwave ammo. Shooting the gas heater canisters is also good. It isn't that hard to shoot the targets themselves with regular ammo if they are running straight toward you or straight away from you. The hardest of all is shooting them as they run side-to-side, but it can be done, and with practice, it becomes easier.
9. No Loose Ends - eliminate all bodyguards after starting the evacuation. At around 12:00, Lang will be at the altar. Kill Lee and Maison in broad daylight to trigger the evacuation. Now Lang will begin running all the way back to his car, giving you about a minute to kill as many bodyguards as you can. Kill Lang right before he gets to his car at around 11:00. Keep killing bodyguards as quickly as you can, and look out for any who are trying to evacuate. If you see a red icon in the objective HUD at the lower left, someone got away, so you will have to restart. This is another difficult, but fun, challenge.
EDIT: WizardSleeve08 and others suggest getting started right away by killing Maison, Lang, and as many bodyguards as you can before Lee arrives in her helicopter. See the comments.
10. Boom Boom - eliminate five targets with explosions within thirty seconds. At the beginning, raise the flag in the labyrinth. At about 9:15, Lee and Lang will start down the main steps of the mansion, followed by their personal bodyguards. At this time, look for guards who are near gas heaters. Shoot at the ground near the heaters to draw them over. Wait for Lee, Lang, and their bodyguards to pass a heater on the west side of the landing. Shoot at the ground to stop them, then shoot at the gas canister. Hopefully you will get at least three of your targets in the explosion. Shoot at the other canisters you set up to get your five kills. This may take some practice. (Thanks again to Kevin Tucker at ShackNews for this tip.)
11. Bodysnatcher - eliminate all of the bodyguards. Self-explanatory and easy. Like with challenge #1, you will want to hide as many bodies as possible to avoid triggering an evacuation.
12. An Elabyrinth Plot - eliminate Dorian Lang and Doris Lee with the statue in the maze. Raise the flag by shooting the crank. Lang and Lee will arrive at the center of the labyrinth at 7:30. When they're both in the statue's shadow, shoot the boards propping up the base of the statue.
13. A Waltz With Death - eliminate seven targets during the bridal waltz. If you do nothing to alter the timeline, the waltz begins at about 4:25. It only lasts for about 20 seconds, and the dance will be cut short if you cause an alarm, so this challenge requires planning and skill if you are soloing it. I watched a lot of videos that had some good ideas, but none of them worked for me. They were all based around changing the timeline and causing distractions, which I found too difficult to do under the tight time constraints. The way I did it was learn which guards were standing still and not being watched by anyone during that crucial 4:25-4:05 timeframe. I shot two of them, then shot the chandelier cord to get the 3 principals, then shot two more bodyguards who were outside the rotunda. Don't get discouraged if you have to retry many times; it's hard. Find some video guides if you like, but be prepared for them to not work for you.
FEATS
14. Agnomaly - [redacted] Shoot the 6 garden gnomes. Unlike the other collectibles, these must be shot in a specific order: 1) to the west of the large bluff in front of you, there are two brick towers. The gnome is on top of the E tower. It will spin to face you after you aim at it. This is the only standing gnome; all the rest are floating on their sides in water. 2) in the W corner of the lilypad pond to the W of the wedding guests. 3) in the W end of the pool behind the photographer's stand, below the waterfall. 4) in the lake, a short distance from the E corner of the grounds. 5) in the northeast pool of the 8 rectangular-ish pools between the parking lot and the mansion, 6) in the large pond to the W of the labyrinth.
15. Hawkeye - shoot all of the hawks. There are three hawks that fly above the mansion. They circle slowly and are fairly easy to hit when they are flying directly toward you or away from you.
16. Art Critic - [redacted] Shoot the painting on the easel, wait for the butler to reset it, and shoot it again.
17. Prometheus - [redacted] Look to the right of the helicopter and notice the cloud of mist. Just to the right of that, there is a pile of logs in front of a small cave. Shoot the logs to start a fire.
18. Faithless - [redacted] To win this challenge, you must kill the bride undetected before she reaches the altar. The quickest way is to take out the bride, her father (Lang), the chauffeur, and the bodyguard before any of them leave the parking lot. Or, you can spare the chauffeur by waiting until the other three pass the first pool and shooting them so that they fall in it. (Anytime I tried to stage an accidental electrocution using the water faucets and lights, the chauffeur always witnessed it, so I had to kill him anyway.) The challenge will be awarded later, when the groom is left at the altar by himself. (EDIT: TangerineGamer says, "basically just shooting the glass on the alter before the ceremony starts is enough to get the challenge." See the comments for a link.)
19. How Do You Due? - [redacted] Shoot the 10 pigeons. They are: 1) in front of the white car in the parking lot (not the limo), hidden in the grass and rocks. This one is hard to spot, but if you look for movement, you will see it. It's also hard to hit. 2) on the cliff below the skydiver, about halfway down. 3) on the extreme W corner of the railing of the 1st floor, 4) on the square part of the W end of the roof where the bodyguard walks up to periodically, 5) next to the spire on top of the rotunda, 6) on the extreme E corner of the 3rd floor (below the top balcony), 7) behind the bag of rice that's standing on its end on the sub-roof at the E end of the mansion (also see Challenge #22). 8) on a railing at the E end of the ground floor. This one is behind one of the 47 "Kiln Time" clay pots. You must shoot the pot to see it. 9) atop the statue at the E corner of the grounds. 10) in a tree S of the labyrinth. Look for a tall clump of tree near the long, straight section of aqueduct S of the labyrinth. It is in full sunlight. Zoom in all the way, scan that clump of tree, and look for movement.
20. Psycho Daisies - shoot the bridal bouquet. The bouquet spawns after the bride exits the limo and despawns when she tosses it after the ceremony. You can shoot it anytime in between, but IMO it's easiest to shoot it immediately after it spawns.
21. Blackout - turn off the lights in all rooms of the mansion. You have to shoot the light switches, which are on rectangular faceplates on the walls. A few are in plain sight, while some are partially obscured or are in dim places that are hard to spot. There are 18 in all: 1 on the ground floor W wing, 1 on the ground floor E wing, 3 on the 1st floor W wing, 1 on the 1st floor center (it's on an exterior wall), 2 on the 1st floor E wing, 3 on the 2nd floor W wing, 1 on the 2nd floor center (behind an NPC's head - be careful!), 1 on the 2nd floor E wing, 3 on the 3rd floor W wing, 1 on the 3rd floor E wing, and 1 in the rotunda. For the last one, you can shoot the chandelier cable instead of the switch.
22. And He Shall Rice - [redacted] Look on the sub-roof at the E end of the mansion. You'll see some junk that may not look like anything, but it's actually birdcages and bags of rice. Shoot the bag of rice that's standing on its end.
23. Quacker Than The Eye - [redacted] Shoot the 10 painted clay ducks. They are: 1) E of the top of the waterfall (purple), 2) in the pool behind and to the E of the photographer stand (green), 3) in the aqueduct on the W side, by the parking lot (white), 4) in the large pool to the W of the labyrinth (navy), 5) beside the gas heater by the NW corner of the labyrinth (blue), 6) in the bushes on the 1st floor level of the mansion, on the W side (yellow), 7) on the roof, below the E skylight (blue), 8) outside the rotunda, E of the chandelier (white), 9) on a shelf near the door of the pantry on the ground floor of the mansion, to the E of the main stairs (red), 10) atop the statue at the N end of the wedding grounds (yellow).
24. Kiln Time - [redacted] Shoot the 47 clay pots on the grounds of the mansion. Most of them are easy to spot. There are six that are partly obscured from your vantage point by the structures and trees of the wedding grounds, but they still aren't hard to find or shoot. If you're two short, the ones you missed are at the photographer's stand. If you're still two short, the ones you missed are at the top of the main stairs.
25. Are You Pondering What I'm Pondering? - [redacted] Shoot the two mice in the pantry (ground floor of the mansion, E of the main stairs). The name of this challenge is from Pinky and the Brain, a Warner Bros. cartoon starring two mice.
26. Take a Hike - [redacted] Simply shoot the barricade preventing the hiker from reaching the waterfall. Easiest challenge on the map.
27. The Higher You Climb - [redacted] Kill the skydiver in the air. To make him jump, shoot his beer bottle. The easiest way to kill him is by firing a shockwave below his feet right when he's jumping off. The timing on this one must be exact, so it will probably take you a few tries.
The clip below demonstrates, in order: Take a Hike, The Higher You Climb, Blackout, How Do You Due, And He Shall Rice, Quacker Than the Eye, Are You Pondering What I'm Pondering, Agnomaly, and Hawkeye. (The only challenge I was completing for the first time is Blackout, so it's the only one that shows the progress and completion indicators in the HUD.)
Sorry, this Game Clip has been removed