To 100%: buy multiple (every?) DLC and solve some ARG-style puzzles. Most achievements are rewarded during regular play (which in this case means idling).

To 100%: collect two sets of collectibles, each in one run, and do a couple of miscellaneous tasks.

To 100%: bring a companion item with you and do a couple of other simple things detailed in the desc. for each achievement.

To 100%: do a few miscellaneous tasks that include changing the system time, leaving the game open for 24 hours straight and running an undocumented console command.

[SEA] Achievements straight-up don't work.

To 100%: buy the DLC, complete every in-game mission and get every collectible, and do a few miscellaneous tasks.