It took me about 6 hours total over two days to get all these done, 4 maps each day. I'll rank them from hardest to easiest based on the number of times it took me to complete each.
1. Subway- This one was easily the hardest of them all, and took me about 10 tries to get. After the first couple retries, the hardest part for me was making that stupid jump between trains. I messed up twice on it, and had to start from the beginning again. Another two times I got to the end, only to have the last couple guys kill me with their magical grenades. Even my final successful run resulted in me taking massive damage from the last fucking enemy, I must have shot him in the head about 200 times before he finally died, and I went from 8 med packs to 3 LOL.
2. Elevator- This one took me about 5 tries. The first time I was overwhelmed by those ghost monsters. The other 3 times I kept dying at the end with the last 4 guys usually one or two shotting me from the elevator. My successful run consisted of me throwing grenades as soon as the elevator doors opened and killing them all.
3. Catacombs- Only annoying part about this is the beginning, which took me about 3 tries to get through. When going down the stairs I would kill almost everyone and then the last guy would one shot me. Successful run came when I kept luring them back to the stairs, going slow mo, and destroying them with the shotgun. In the church I used the Type 12 until it ran out, then the Cannon, and before the guys spawned through the doors, I jumped behind the altar and took them out with the ASP Rifle.
4. Garage- Took me 3 tries. I died twice at the end before I was able to clear out all the enemies. Third time I destroyed them with the ASP Rifle, then ran into the middle to avoid the chopper. When I saw the chopper fly overhead, I went into Slow Mo and unloaded on it with the Cannon and then the ASP Rifle afterward.
5. Sanitarium- Took me 3 tries. The worst section here is the middle, when the enemies start busting through both doors, more enemies spawn in the windows on the other side, and then a heavy spawns. I just lobbed my grenades at the two doors, sniped the guys in the other window (there were 3, and then another 3 spawn when the heavy does), and for the Heavy I threw down a turret and shot him in Slow Mo with the Assault Rifle.
6. Clinic- Got it on my first try. Killed the first couple guys with my pistols, then killed the Heavy with the shotgun in Slow Mo, and killed everyone else with either the Penetrator, Assault Rifle, or the Type 12 (once I picked it up).
7. Arena- Annihilated this one with a combination of the Type 7, VES, and the Assault Rifle. The two mechs were easy since they spawn so far away and always seem to get stuck in the rubble. Throwing the turrets down on either side makes killing the enemies even easier.
8. Sprint- The easiest of them all. Just keep killing all the enemies with whatever weapon you have, as soon as you clear them out they'll be a bunch of boosters, armor, and weapons to replenish you. I took out the Heavy on the stairs with the ASP Rifle then switched to the Type 7. Picked up the Type 12 and used that to clear enemies until I got to the Grenade Launcher which I picked up so I could kill the Shield guy. Once he went down I switched between the Type 7 and 12 until every enemy was toast, then ran to the elevator and watched the achievement unlock.
All in all, besides the Subway, I really didn't think they were that hard, and most of them were much easier then Distribution and Vault in the original FEAR.