Achievements, Payday 2 – Achievements Meaning: ‘Željezo se kuje dok je vru?e. ?ega nema, ne može se ni uzeti’ in Payday 2

The update notes say:

• Added 4 more achievements along the same lines as the "Medium Rare" achievement added last week
• Renamed "Medium Rare" achievement to "Medium Well"

Since they are along the same lines as the previous achievement, they mean the player should complete both the light world and the dark world of a chapter, and the sum of the best times for that chapter should be lower than some threshold. Here are the descriptions of each one:

• Rare - Spend as little time as possible in The Forest (around 320 seconds)
• Medium Rare - Spend as little time as possible in The Hospital (around 430 or 460 or 490 seconds)
• Medium - Spend as little time as possible in The Salt Factory (around 570 seconds)
• Medium Well - Spend as little time as possible in Hell (around 500 or 550 seconds)
• Well Done - Spend as little time as possible in The Rapture (around 740 seconds or more)

About the old secret achievements:

• Suffragette - Complete the light world of the Cotton Alley
• Seneca Falls - Complete the dark world of the Cotton Alley
• Well look at you! - Input the petaphile code to play with Tofu Boy

Sources:

• Super Meat Boy Wikia
• SteamStats
• Steam Forums

Note about the times for the achievements: Many people use Super Meat Boy Stats (old repo at Google Code, new repo at GitHub) program to find out how much time they have spent on each chapter. However, the reported total time is not exactly accurate because "it counts 15 secs per completed warp zone", and people posting their total times didn't mention if the warp zone time has been subtracted or not. For this reason, the times I posted above might be quite inaccurate.

I got both of these during story missions.
The car one I got in the first mission where you have the cars (shrouded bunker), there a part where you enter a porch overlooking a room, with a short spiral staircase on the right, enemies tend to bottleneck just after the stairs, you can hide behind the porch's rail and send cars down the stairs, with some luck you'd get to the enemies before they blow up the cars and blow up 3-5 of them. I managed to do this on Nightmare after a few tries.
I got the arrow one during the prison break mission just after releasing the prisoner, there was a a part when he's busy hacking and you have to fend off enemies, a few guards grouped near the left side of this area and I shot one with an arrow and it got the job done. A better opportunity was in Dead City Reverse, there was a part where you watch a few soldiers fight off mutants, while they were busy with the mutants, I used my arrows and blew them all up.
If you're past these missions, I recommend doing this either on Bash TV, sewer missions (DLC) or later in the game in the distillery missions, there will be parts where you're on a ledge and multiple mutants will climb over, I think this will be a good place to try the trophies although I didn't get them there myself.