Hello achievement hunters! On July 11, 2019, Ubisoft updated the requirement to unlock the "Master" achievement.
Previously: Master - Complete a Terrorist Hunt mission on REALISTIC difficulty.
As of July 2019: Master - Achieve Gold Rank
This achievement must now be completed by playing ranked multiplayer and ranking up. The ranks available in Rainbow Six Siege are: Copper IV, III, II, I, Bronze IV, III, II, I, Silver IV, III, II, I, Gold IV, III, II, I, Platinum III, II, I, and lastly, Diamond.
For those of you having trouble getting a good team together or are sick of playing with randoms, here's a really easy way to get this achievement solo. I take no credit for the idea but wanted to share the solution. I'll link the video that helped me below.
Difficulty: Realistic (obviously)
Game Mode: Hostage Extraction
TL;DR version - scout ahead with drone, clear out the room with the hostage, extract hostage to rooftop, go prone and snipe terrorists in the extraction zone, snipe terrorists as they come out of the building to shoot you. When clear, get hostage to extraction zone. Win.
Long version - When the game starts, scout ahead with your drone and locate the hostage. If the hostage is in the master bedroom, construction room, or dining room you should be good. If the hostage is in the garage area I'd restart. I didn't make it far with the hostage in the garage due to being on the ground floor and getting flanked a lot. For the other 3 locations, HIDE YOUR DRONE in the room with the hostage.
Master bedroom - hop up on the ledge outside the large window and clear the room making sure to use your drone to spot enemies inside. Watch out for the small window to the left of the big window! There may be enemies in that room too.
Construction room - rappel up the front of the house and clear the room through the front window making sure to use your drone to spot enemies inside.
Dining room - rappel up the side of the house to the window above the garage. Clear the room through the window making sure to use your drone to spot enemies inside. Be careful of the small room to the right of the overturned dining table as well as enemies running into and out of the kitchen. May want to toggle on Glaz's zoom scope to take out anyone in the kitchen.
Once the room is clear, refill your ammo (dining room, construction room), then grab the hostage and get the hell out of there by rappelling up onto the roof through the window you used to enter. I'm not sure how the game selects where the extraction zone is but I had the most luck when the extraction zone was the construction site like in the video below. Release the hostage in a safe spot, go prone, and peek over the edge of the roof to snipe every terrorist in the extraction zone. Once they're cleared out prepare for another 15+ terrorists to stream out of the house looking for you. Peek and snipe them all too. All return fire should be hitting the roof and not you. Occasionally I got hit but it was usually because I was peeking out too far. You should have another drone so use it to help spot terrorists below that you can't see from the roof. Once everyone is cleared out, grab the hostage, rappel down and extract to safety. Nice work!
Again I take no credit for this idea. This is the video that helped me out. It's in French and on PS4 but you get the idea: