Foxxiest of the Hounds achievement requires to finish the game without sounding an alarm. Alarms are sounded by enemies activating an alarm panel or security systems going "Hostile". Security systems going "Suspicious" don't count as an alarm so getting spotted for a split second won't affect your achievement. Also, getting spotted by an enemy or more but without giving them a chance to sound the alarm also won't affect your achievement. The way it works is that during certain main story missions, so side quests don't count, you can get an exerience bonus with the title "Smooth Operator". The problem with Smooth Operator is that for example leaving a dead body where other guards may find it, sounding the alarm, will affect your achievement even if the alarm is given AFTER getting Smooth Operator. Just make sure to hide yuour victims. By reading the missions or quests tab you will notice they are marked with letters and numbers like M1, M2, M3 and so on. These are main story missions.
List of missions to get Smooth Operator on:
Detroit Police Station M5 - Investigating the Suicide Terrorist 1. Retrieve the dead terrorist's Neural Hub
Highland Park M10 - Following the clues in Highland Park 2. Infiltrate the Mercenary Compound 3. Locate the Mercenary Leader
Tai Yong Medical M1 - Searching for Proof 4. Enter TYM's tower by passing through the Pangu 5. Find and view Van Bruggen's recording
M2 - Entering the Dragon's Lair 6. Confront Zhao in the Penthouse 7. Escape and get to the hangar 8. Open the hangar bay doors
Picus M1 - Confronting Eliza Cassan 9. Escape the ambush by reaching a lower floor 10. Get to the funicular 11. Summon the funicular 12. Get inside room 802-11
Detroit - during your second visit M2 - Finding Isaias Sandoval 13. Obtain Sandoval's location from Taggart. 14. Find and confront Sandoval
Hengsha - during your second visit M3 - You scratch my back, I'll scratch yours ( ONLY PART OF DLC ) 15. Rescue Tong's son
M4 - Stowing Away 16. Retrieve Tong's Package from the shed 17. Place the C4 on Administrator Wang's desk
Omega Ranch M1 - Rescuing Megan and her team 18. Search for Signs of Megan's Team 19. Disable the Signal Jammer 20. Upload a virus to the security computer and enter the restricted area
Panchaea M1 - Shutting Down Darrow's Signal 21. Get inside the tower and disengage the lockdown 22. Reach the Broadcast Center
As you can see there are 22 main story mission objectives that require Smooth Operator. However, some of them excluded by finishing the objective by talking your way and so gaining a different experience bonus, "Silver Tongue". An example of this is mission M5 - 1 during your first visit to Detroit, at the start of the game, Investigating the suicide bomber. By getting Silver Tongue you will not have 21 objectives (22 with the DLC) but 20 (21 with DLC)/ This will not affect you in any way towards the achievement.
Point of this achievement is to not kill or get anyone killed by personal actions. As such, knocking someone unconcious and dropping him off the building, balcony or in something like electrified floors will count as killing him.
IMPORTANT! Not killing anyone is required from the start of the game, so including the lab attack at the very start, and in certain quests that ask you to. The quests that ask you to kill someone can be done by either steering the conversation towards a different method or knocking them out if it's not necessary to kill them. Mission spoiler! As an example of the above, in one of the side missions in Detroit you're asked to "deal" with two drug pushers. There's even a conversation option "Kill them?". Knocking them out still counts as them being dealt with.
Minor spoiler alert! At the start of your first mission you get the choice during your conversation with Sarif inside the helicopter for "Lethal" or "Non Lethal", followed "Long Range" or "Close Range". These offer a choice for the first weapon you get. Trying to get this achievement the first option is obvious, "Non Lethal". Second set of options is however a bit trickier but going for "Long Range" should be the choice. Reason behind this is that you get the Tranquilizer Rifle instead of the Taser Gun. Why should you choose the Tranq Rifle? Easy. It's a bit harder to find and with a bit of effort you can even use it at close range. Not to get confused but you should get a Taser Gun as a secondary weapon. The Taser Gun knocks enemies out faster that the Tranq Rifle and can be used a lot easier behind corners during cover.
Boss fights don't count against the Pacifist achievement. So by all means remember to get at least a 3rd weapon, upgrade it and the rest should be obvious.
Be careful with the Tranq Rifle. Only happened to me twice but it did. After you stun an enemy using the Tranq Rifle you should inspect and make sure the pictogram over its body shows Zzzs not a skull. Although I noticed that both times it happened after I executed a headshot just check them out. Tedious but can never be too careful. Taser Gun doesn't seem to cause any problems.
The last of the non-lethal options is using PEPS. I for one did not but it can prove usefull. It's mostly a matter of choice and of course inventory management.
There seems to be a glitch that causes knocked out opponents dragged inside vents to die. I would stay away from them and use a room like a closet or one with no or few windows. (Courtesy of Duke Voodoo).
To get these two achievements you will of course need to stay out of sight most of the time. As such, finding side paths or secret routes to your destination is always a good idea. For this reason, I found getting the following Augmentations best:
Hacking: Capture (Got it to 3 ASAP to be able to hack mostly everthing in Detroit, and getting an experience boost by hacking the offices inside Sariff Industries is certainly worth it. Also my second priority was being able to turn cameras off)
Hacking: Stealth (Although a usefull Aug, I kept it at level 2 out of 3 for quite some time)
Wayfinder Radar System
Icarus Landing System ( You won't be needing this too many times but you'll be glad you have it)
Cybernetic Leg Prosthesis (Jumping higher, running and landing silently with low energy consumption. What more can you ask)
Sarif Series 8 Energy Converter (decided to go full since cloaking even fully upgraded is an energy hog)
To note that this is not a must. Fortunately, there's quite a bit of freedom when it comes to the Augmentation you can choose so feel free to experiment. I would stay away from Aim Stabilizer Aug though. I didn't encounter any need to run and shoot. Getting Chemical Resistance, from the Health System tree, and EMP Shielding from the Dermal Armor tree did prove usefull on certain ocassions. The tip during the loading screens about not being able to get all the upgrades is true. However, you will be able to get most of them eventually so don't be afraid to try out. You can always save before getting an upgrade and load if you find it less than usefull.
Being similar in approach, getting Foxiest of the Hounds and Pacifist during the same playthrough is not necessary but I did find it adequate.
Hacking Alarm Panels, green palm scanners on walls, can prove very usefull but I wouldn't relly on hacking them all and start shooting. What if you missed one?
Going overboard and trying knocking every enemy out is not the way to go about it. You can certainly try though.
Using Gas Grenades against human opponents will knock them out not kill them. However, using an EMP grenade against a robot will not count as a kill. (untested)
Hacking, capturing and turning a sentry or a robot against opponents counts as a kill and will lock you out of the achievement.
Just make sure you save often, try to evade opponents or just knock them out and hide them somewhere safe.
During the 4th boss fight, fighting Zhao, killing the soldiers counts as a kill, BUT killing the 3 girls in the middle, the ones connected to the machine by hacking it does not count against the achievement.
As hard as these two achievements sound, they can certainly be done.