In this article I will teach you about some Black Ops achievements I have earned. This is NOT all of them. I will include guides to harder ones when requested. May I also say that this is the way I GOT THEM (Theres always a troll out there).
Sources:
Youtube.com
callofduty.wikia.com
callofduty.com
Google.com/Images

Step 1: Sally Likes Blood

This is a harder one. First of all it is worth 15 gamerscore.
Description : Demonstrate killer economic sensibilities by taking down 3 enemies with a single bullet.
Walkthrough: 1. Get to the campaign mission "Numbers"
2. After the epic slo-mo jump, when Clarke opens the secret stash, get the PSG1.
3. As soon as Clarke gets kicks the next door open, take 1 step in and crouch.
4. There should be 3 Spetnaz that line up on the next stairway.
Note: This may take several tries, so don't get upset if it doesn't work the first time.
Video: This is not mine < http://www.youtube.com/watch?v=_7lAl4JYfKg >

Step 2: Pathfinder

This is an easy 50 gamerscore.
Description : Guide the squad through the Soviet outpost without them getting killed.
Walkthrough : 1. Go to the mission WMD, best on recruit.
2. After the blackbird scene, simply command the squad into the house.
3. If you die in the skirmish in the first safehouse, you won't get the achievement.
4. Don't forget to notice the guys out back during the escape from safehouse 1.
5. When you enter the Barracks, stay in cover. Continue to use your flashbangs, and grenades.
6. When planting the C4, make sure you have some distance.
Note : You don't get it until the cutscene afterwords, so don't freak out.
Video: This video is not mine < http://www.youtube.com/watch?v=v2PdBkaaq6w >

Step 3: They Are Going THROUGH and the Eagle Has Landers.

These are 2 very easy achivements, and can easily be obtained in the same game, for ease of access.
They are going THROUGH!
Description : In Ascension, kill at least 5 zombies with 1 Gersh Device
Walkthrough: 1. First, get the Gersh Device from the mystery box.
2. Get to the PhD Flopper Area.
3. Get multiple zombies chasing you in a circle
4. As you run, prep a Gersh Device until you are ready to throw it.
5. It lasts for about five seconds and sucks all zombies closer to it.
The Eagle has Landers :
Description : In Ascension, escape on all 3 lunar landers(in one game).
Walkthrough: 1. You should have turned on the power.
2. Get to PhD. Flopper
3. Call the lander and use it.
4. Go to the power room again.
5. Open the door straight foward from the door.
6. Go down the stairs, and turn left, and go up more stairs.
7. Open the door and use the lunar lander.
8. Go the the upstairs power room, and open the door
9. Turn left, and then turn right. Go to the back of that junkyard.
10. Call the lander and use it.
11. You should get the achivement while on the lander.
Videos: They are going THROUGH : http://www.youtube.com/watch?v=0cAhGuV4d-4
The Eagle has landers: This isn't mine http://www.youtube.com/watch?v=OyFnIqyK0uU
and subscribe to that guy's videos