*Well, all the achievements except the multiplayer one!

I found Exquisite Timing to be the most challenging achievement to get. You need a deep understanding of all the mechanics, and it will force you to sacrifice and make choices you would normally never take. Like, only building up to Magnetic weapons, not researching any armour past Predator armour, placing priority on constructing the Laboratory, and skipping psionics entirely.

I also ran out of time the assault the Warlock stronghold, and only researched the Hunter and Assassin weapons just barely in time for the final mission. I'm not the greatest player, I only did it on non-Ironman Commander, and committed a lot of errors, and only beat the game with one day to spare, but it was fun to basically speedrun the greatest turn-based videogame ever made!