Left 4 Dead

For this achievement you must simply finish the campaign.
You do not have to necessarily play the whole campaign as you can join one in progress and still unlock the achievement. Please note that you can die countless times throughout but on the final sequence (when you have called for rescue) if you are killed you can no longer re-join.

If you are really struggling just select the easiest difficulty and play on single player, as there is no lag and the AI you play with can often spot infected that are nearly impossible to see.

For this achievement, you must survive the "No Mercy" campaign. You have to reach the escape vehicle and not die at the last part. If you die, you will not get this achievement.

This is not a hard achievement, but if you're struggling then change the difficulty to Easy. This can be done with AI players. I did most of the achievements with AI players. So it is possible. You can join a session in progress and reach the end. You will still unlock the achievement this way.