Games

Shadows of Doubt. These achievements are not rare because they are difficult, but because they are tedious. Most of the ones listed here are just repeatedly eating a food item, which costs almost nothing (1200 credits for solving a typical case, 700 for an easy case; 12 credits to buy a Croque Monsieur, the most expensive food in the game.)

Rat Catcher: Solve a murder while your PoV is at the ground, but you can crawl up walls, and also nobody will talk to you. This achievement is rare mostly because this game mode was added a few years after this game peaked in popularity.

Spare No One: KO everyone in town, in a game where you are discouraged from using violence. Finding a katana and knocking out everyone you see on the street is easy enough. The real challenge here is keeping track of everyone. Some NPCs glitch out and get stuck in their houses, never go outside, and never come to work. You'll have to identify them, find out where they live, and hunt them down. Some NPCs have accented characters in their names, making them difficult to search for on the computers in game, which do not allow you to type such characters.

If you make a case board to keep track of everyone, the case board takes exponentially increasing time to open for each person you pin onto it. By the end, it was taking me about 5 seconds each time I opened it. That was in a custom procedurally generated town of the smallest possible size -- it would be worse on any of the stock towns.

Out of the Red: pay off 10,000 to a loan shark in 1,000 credit increments, one payment per day. Raising 10,000 can be done in half a day easily. Paying it off takes for ever. You can't make payments early. Each one-day wait takes a few minutes, since the game has to simulate every NPC in town. And remember to turn off the procedurally generated murders, because otherwise your loan shark might get killed, and you can't make payments any more (it happened to me, and I had to load a save.)