Left 4 Dead

This is easiest if you be sure to have a pipe bomb.
To start the finale of chapter 2, you have to start up a generator by holding down X until the meter is full. Then, it plays out like a normal finale, with a few waves of zombies and then a Tank. After the Tank, it will be quiet for a few moments for you to regroup. Then the next wave of zombies will attack. About 30 seconds or so into the second wave, the generator will turn off and you will have to restart it.
It's best if your team is holding out on the ledge right above the generator switch. When it turns off, someone throw their pipebomb to distract the zombies while another person jumps down right away and restarts the generator. You will have to hold down X just like the first time.
Only the person that actually turns it back on gets the achievement, so you'll have to play through again if the rest of your team needs it.

In order to get the “Quick Power” achievement you will need to have the “Crash Course” DLC. Play through the second chapter until you reach the setion where you need to power up the generator. Once you have done this it will start up the finale. Be prepared for a couple of waves of zombies followed by a Tank. As soon as you have killed the Tank head towards the generator again and you will find it will all go dark and you have to restart the generator. You now have 30 seconds to restart this to gain this achievement. If you are playing through on EASY then this should be pretty straight forward. I personally found that if you get to it quick enough you should have ample time to restart it before you get hit by the next wave of zombies. While you are restarting the generator have your team-mates sound you to protect you from any oncoming zombies. After the generator has restarted the achievement will pop up.