Void Star, one needs to complete all 16 maps with full 3-star ratings. Usually simply getting enough points is easy, but at least on couple maps even that will take some serious effort.
Mrs. Pilsen's Remorse is probably the easiest achievement in the DLC. You merely need to go into each map and pick up Emily's hidden doll. Of course, finding them all without a guide might take awhile but who has time for that...
Your goal in Mystery Foe is to assassinate a target who is not clear from the start, similarly to the main game mission, Lady Boyle's Last Party. There are four clues that will cut down the number of possible victims. The final one informs you of exactly who you need to eliminate. Staying completely unnoticed and a faster completion increase your points. You also get a bonus for killing your target before picking up all the clues.
I completed the Normal version of this map at 3 stars when I first played the game in 2015. I recall the challenge being fairly difficult. But when I did the Expert version now -- which has very hard stealth difficulty and clues need to be pickpocketed from people -- I got 3 stars on my second attempt.
Maybe it was the achievement that took time back then, as for Natural Talent you have to complete the challenge without using the four supernatural powers you are given. Without Blink stealth becomes so much more difficult.
This map's achievement, Rare Collector, also needs to be unlocked on Expert which has 16 collectibles to gather. Since score is irrelevant, you can simply take out the guards first, although they seem to start respawning and continue bothering you. The challenge ends if you are spotted 3 times.
Back Alley Brawl
The challenge map itself is pretty simple: killing varying enemy waves until an infinite wave 13 that starts with a boss fight against Daud. The achievement, By My Hand Alone, requires you to get there after having killed every single enemy yourself. If you do not have 53 kills at the start of the final wave, you will not be getting the achievement.
What makes it so difficult is the amount of things that can go wrong. The enemies are -- apart from few guaranteed spawns -- random types from the main game and thus from 4 different factions (city guard, assassins, gang members, and weepers) that will rush into killing each other. The random spawn points do not help either.
Even on the first wave you can already get screwed if two guards and a weeper spawn at the far end of the alley. The weeper will die to two sword swings. I suppose a very quick and accurate crossbow shot could save the kills for you but you might as well restart right away.
Things that can go wrong also include the power upgrades that you get by picking up a rune that spawn at a random location after certain waves. The one that spawns after wave 2 is Devouring Swarm which is useless for the purposes of getting the achievement. You still need to pick it up, though, as when the much more useful Bend Time rune spawns after wave 5, the first rune you pick up -- regardless of which one -- will be Devouring Swarm and you will need to go get the second one too.
If the first one spawns on top of one of the two far roofs, you will be in trouble if you try to go get it between the waves. Instead, you need to leave the last enemy of the next wave alive and go get the rune without hurry.
After getting Bend Time, killing stuff gets bit easier. I found that standing in the middle of the map by the tree and looking at the alley end where you start will give you a good idea where enemies spawn at the start of each wave. And for actual killing stuff, double jumping (since you have Agility upgrade from the beginning) over the enemies for drop assassination is the most efficient way. I wonder if that is a possible technique in the second game still. It feels somewhat cheesy, I must hesitantly admit.
On waves 10 and 11 you will need to watch out for a spawning tallboy since their exploding arrows seem to one-shot other factions' members. Killing them first is a good tactic but you can possibly also manage to kill the other enemies during Bend Time. Wave 12 has two tallboys and I think you really need to go for the other 3 opponents first. The tallboys will be at opposite ends of the alley, and one Bend Time will not be enough to kill them both before an arrow gets fired. I came to that conclusion after failing the wave twice after otherwise perfect runs.
And perfect is the keyword here. You truly need to have perfect play under random circumstances to unlock By My Hand Alone. The satisfaction I got from seeing it finally pop up was pretty immense.
The map's achievement, Headhunter, requires headshots and 100% accuracy. The difficult rooms for this are #4 where you cannot stealth from the two assassins spawning (headshotting them in combat is fairly challenging), and room #5 where you need to kill two targets from a long distance. One of them is walking up a spiral staircase, requiring you to lead your shot a bit.
The final room with 3 assassins would also be difficult but luckily there is bit of an exploit there. Getting on the top of the bookshelf at the opposite side of the room from where you enter and then Blinking towards the wall will trigger the room exit and finish the challenge.
Bend Time Massacre
Time Management requires you to complete either Bend Time Massacre or Kill Chain without failing any wave (including bonus waves). I did the achievement on this one by following a video guide on YouTube. It is kind of a fun challenge, though.
Daredevil. Hell, even getting 3 stars on this map is a pain in the ass.
The goal in Bonfires is to reach a beam of light before timer hits zero. Hitting the beam gives you some seconds back to get to the next one.
The difficulty comes from the beam spawn order being random after the first two. You cannot train one set path but instead have to learn the whole map -- what is the fastest route from every possible beam location to the rest. And you constantly need to make the absolutely maximum range double jump blink combos if you want to continue the run.
There are also six "tricky jumps" in the map that give you extra 1000 points per beam. They are not marked any way and you only get to know you made one when hitting the beam. I imagine it would take an eternity to figure out where those spots are on your own. To unlock Daredevil, you need to execute each tricky jump within one round. With you following a random route and how you can barely even keep up with the timer at first, the achievement is indeed quite the challenge.
The Expert version ramps up the difficulty by not giving you endless mana -- you have to learn the spots where mana pickups spawn as well. And that is not all! There are also assassins in the map who just love to use their Pull ability on you.
I had one really frustrating moment on Expert when I was at 29850 points (30k is required for 3 stars). I was at the height of a double jump about to blink a tricky jump to a beam with couple seconds left on the timer when an assassin grabs me from mid-air with Pull and drops me into the Void...
This map's achievement, Assassin vs Machine, requires you to reach the end before the train, which happens after about 34 seconds. It is possible to do so by just blinking and jumping the route but it is evidently easier to blink aboard the train halfway and then at the end blink to the exit before train has stopped.
This is in fact a required tactic on the Expert version where you are only given enough mana for a single Blink at a time. Although there is also a helpful glitch -- restarting right after blinking will increase your maximum mana to make two consecutive blinks. The added assassins on Expert do not really make a difference but you do need to use your pistol to destroy the wall of light that is added on the train tracks.
Long Way Down achievement. You must differ from the lit path. I got the stars and achievement with a 3 assassinations tactic.
The Expert version adds fall damage and some obstacles but the exact same route still applied for me. I also saw an absolutely ridiculous video of someone doing a one dropkill run. Apparently bouncing off an enemy does not count as landing.
Even now, after having mastered the Trials, I hope Dishonored 2 will not have a similar piece of DLC. I do not think many want such. They are not very interesting additions to games like these.