Not so much an achievement I'm planning to ever top, but it is the hardest achievement in any game I own: Star Ocean IV: The Last Hope – Obtain 100% of all battle trophies.
Let's just say that these trophies are really insane sometimes. For starters you have trophies in there that require you to get exact amounts of damage (like hit 7,777 or heal 77,777). These are very much luck based so don't even bet on them, but there are online tips where your odds are bigger to score them. Others require insane skill, like beating the post-game mega boss in less than 10 minutes. But the grinding trophies, oh boy, those are hell. Each character has these, and while some seem easier (jump 5,555 times), others are just a straight pain in the butt (defeat 30,000 enemies), alongside each character having to complete 100 battles in the colosseum (with some consecutive as well).
There are 900 total battle trophies, 100 for each playable character. For all the "per battle" trophies, such as consecutive hits or exact damage, you will personally have to do the action. For some of the larger accumulated ones, such as fighting a certain number of battles, those can be worked on by the AI.
Most achievement sites where you check on this game will only have a handful of people with 100% at most.
If you feel like throwing away your life on getting a 100%, this is your game.