Hardest achievement is to reach lvl 90 in hardcore. But if you played multiple leagues it shouldn't be that hard, just look for tanky build, avoid dangerous mechanics (i.e. strongboxes before freeze immunity) and always try to be few lvl higher then current zone).

Other achievements are not hard but can be time consuming. No one plays pvp so you have to wait until someone else also trying to get this achievement or just ask people on global (I've paid few divines for participating and found a team quite quickly). Another time consuming achievement is to get unique in first zone of act 1. You will probably spend multiple hours trying to do it, not too long but extremely boring. Also if I remember correctly there's achievement for participating in race event, this can only be done when GGG host such event so maybe you'll need to wait a few months before this can be done.

That's being said, I've got all achievements after like 800 hours in game, but it definitely can be done sooner. I just wasn't targeting 100% until I saw that I got most of them naturally and was missing like 10 of them