50 Cent: Blood On The Sand

Just play through the training level (level 1) and it will unlock I am pretty sure it is impossible to miss this one.

A scenario seems to be when it gives you an optional objective(like kill both snipers) and gives you a short amount of time to do so. Complete one of these objectives in the indicated time, get the bonus points, and unlock the achievement. Most of the time your objective will be highlighted in red to make it easier to see who you should be shooting at!

I'm not sure why this achievement glitched on me, but I can say what I did that could have caused it to glitch.
I started up the game and was going to join random games for the coop achievements. The first game I joined, the Host quit out right away.
The second game I joined I was spectating and it kept saying, teleporting. The person I was spectating Completed a Scenario so I'm thinking, maybe that's why it glitched because I was spectating when the 1st scenario unlocked.

So, if you don't want this easy 5G achievement glitching on you, start up a game by yourself and play until you complete your first scenario and the achievement should unlock. Read KARMAgoesHARD's solution to know what a scenario is.

I would also like to point out that if this achievement does glitch on you, there is no way you can fix it. I deleted my save and it still had my stats up, so I tried starting it on a different Xbox and it had my stats still.