Left 4 Dead 2

Solo with second controller

This is the quickest way I found to collect cans.

Start a No Mercy - Rooftop, scavenge match with your second controller on the infected side, you will still have AI specials spawning but your AI team should be good to help you out, they generally stick to you.

On this map 21 cans spawn and they are all in a very small area. If you know the finale area, you'll know what I mean. Sometimes you might only get 9 or 10, once you get going you can even get all 21 if you get a bit lucky. Just keep playing this scenario and racking up the cans.

When your turn ends you can even use your second controller to pick up cans and get some progress to the 25 drops with a special with your main profile, alternatively just hurl him off of a building to end the round quickly.

Obviously this is also the best strategy for soloing 15 cans in a round.

Rack 'em up!

Easiest way to get this and the "15 gas cans in one match" achievement is to have two people and one guest. I think the last level on Dead Center is the easiest to do this. Have two people be survivors and the guest be infected. Don't do anything with the guest. Be sure to get a health pack before leaving the starting area. Have one person throw all the gas cans down so the other person pouring the cans can get them faster. The person throwing the cans down need to make sure that when the time is about to run out that they are holding on to a gas can. This makes it go into overtime. Just keep holding on to the gas can until the timer starts up again. Dropping the can ends the game. It will take 7 matches to get the full 100 gas cans.

Exactly as it sounds, collect 100 gas cans in Scavenge mode. Grabbing them isn't enough, they aren't counted as "collected" until they're poured. This will just come over time. If you play the Scavenge mode enough, you will probably be able to manage 3-4 gas cans per round. Of course, this all depends on the skill of the player.

I found that its easiest to use two controllers (it was for me) just go to the park and get your supplies, try to go for the farthest gas cans first but just get around the big hedge ring and throw the gas cans in there then go for the closer ones but by now youve only got like 10-20 seconds left so grab a gas can and it doesnt matter if it goes into overtime head for the generator and dump the gas can in , it gets pretty easy to get around 15 gas cans a round because for every gas can you put in he generator you get an extra 20 seconds added to the timer even if you are in overtime