Left 4 Dead 2

This can be done in one round if you have a friend to boost with. Both people need to have a guest and put them both on the survivors team. Have both of your guests pick up a gas can and find somewhere that is fairly safe from a hoard like a corner or small room. Then try to use one of the guests to kill both of the bots and make sure that one of your guests is always holding a gas can. Most likely at this time it will be overtime and as long as one of you is holding a gas can you can keep on going as long as you want. Once the bots are dead get your guests close to each other. The best infected for this is the jockey or the smoker so you can keep killing the special infected until you get one but make sure your guests have a gas can at all times. When you have the infected you want focus on one of the guest survivors at a time. One person grab one while the other uses there guest controller to hit them off then pick up the gas can again and keep doing that 25 times for each person trying to get the achievement.

NOTE: All Scavenge achievements can be done with two controllers in System Link.

Similar to the guide on causing gas can drops, follow it the same way, except instead of letting time run out, try and get as many drops as possible during a single round. I was able to get around 3-4 per round. Just pick the gas tank up after dropping it (your CPU team will annihilate your infected controller, so no worries on dying).

Simply start a Private Scavenge game with a 2nd Controller. (I used the Port on the Passing Campaign), Let your 2nd Pad be Human and you'll play as a Infected. So you let take your 2nd pad an Gascan and put him in front of the generator. Than use your infected (the best one for this is the Smoker) and hit him that he'll lose the Gascan, the A.I.'s will kill you or cut your tongue. Now you let take the 2nd Pad the same Gascan again an use to hit him Again. So you can do this 3 - 5 time a game and your Achievement will unlock after some games