Left 4 Dead 2

Okay guys here is a fool-proof achievement guide, basically guaranteed to get the achievement first (or maybe second) attempt.
You can actually do this with bots by the way so this method is INCREDIBLY effective! TRUST ME AND TRY IT =)
What you want to do is load up the Dead Center campaign on survival, choose the only option there is.

From then on use this video below to guide you, it'll take you to a special place where it's very easy to camp your way to 4 minutes with a melee weapon.

You should not have a difficult time with this solution if you follow the video =)

If it gets to around 3 minutes and it looks like the rest of your team is f**ked then leave them and jump down to the floor below, then just run around until 4 minutes. This is what I did, and I actually did it on my own too.

I agree with all of the above solutions, but I have a few bits to add.

First, if you have already gotten a bronze medal on a level in survival using weapons, you will not be able to get this achievement on that level in 4 minutes (bronze medal). The idea behind that is you have already gotten the medal using weapons other than the melee weapons. So you can try for the seven minute mark for the silver medal with melee weapons only, but that will be much more difficult so keep this in mind.

Second, this achievement was much easier for me and my friends to achieve when there was 2 human survivors and two AI survivors. We tried with three humans and it was unreasonably difficult, 2 seemed to be much easier.

Third my strategy was on bus depot in the Parish. We started by setting up all of the gas and propane tanks in the narrow walkways between barriers parallel to the scaffolding with the button. This was in prep for the tanks, we could fall back around the 3.5 minute mark through these "hallways" firing on the cans and propane tanks creating a wall of fire and explosions to slow down and damage the tanks to make it to 4 minutes. We also setup the explosive and incendiary ammo (found at the entrance to the bus station) on either side of the bus which is parked perpendicular to the bus station. Each grab a molotov (for the tanks) and start it up.

Easy to get. Go to the bus depot on the parish. Get the police baton and hold up near the button. throw a pipe-bomb or Molotov whenever the horde comes and the AI can easily dispose of the tank in the wide open space. I got 5 minutes on my first try. If you get jockeyed or smokered off, just run to the ladder. If you stay near the AI there shouldn't be any problems.
If the AI start acting like retards, get a couple of friends. It is best if they either have the M16 or Sniper Rifle to save you from smokers and jockeys. Your only real problem will be the Tanks. Have your friends open fire on him and you'll be fine.
P.S. If you get incapacitated, DON'T FIRE YOUR PISTOL!!!

i got it in the parish- the bridge map. i played with three ai characters. all i did was i got myself a machete and i hung out at the helicopter, kind of in between the helicopter and the water. the ai would take care of all the zombies, except for a handfull of them that would get past them. inside of the helicopter there are two extra first aid kits if you need them. i used a pipe bomb (which dosent disable the achievement) and a shot to get to my ai teammates and heal them up after they all got downed by the second tank. the ai did most of the damage on the tanks, but i still had to finish off the second one on my own. the one thing to look out for if hanging out where i did is that the zombies that come at you on the opposite side of the helicopter's front end from where you are standing will duck down to get on the same side as you, so you will have to aim low to kill them.

What I did was go in the Dead Center campaign on survival and went in the metal door on the 3rd level and literally only encountered 4 infected and the door is so heavy that only the tank can break thought it. I did this with all cpu and the tank never got me, 5 mins.

A great place to do this achievement is in the Hard Rain campaign, on the sugar mill. There is a locker room near the other end of the map that a team can hide and easily defend in. Next is to make sure you have a team with mics for constant communication, this is a must! As the person going for this achievement, find and grab the katana as it's fast and always a one hit kill, well the rest of your team grabs firearms with laser sights.

To set up your "stronghold" bring both the explosive ammo and fire ammo and deploy them on the table. Everyone then needs health packs, pills or shots(pills are better since your camping it out), and bile or pipe bombs. There is a propane and gas tank ... get both of those out of that room, they are your enemy but use them as a defense measure outside the one entrance into your room. Never leave the room even when the tank comes, 4 human plays can defeat the tank before it incapacitates anyone.

When everyone is ready, the melee person hides behind the lockers but out far enough to melee lucky infected that are missed by bullet fire. Two of the other team mates stay in the "stronghold" well the third runs to the radio, signals and gets back to the room before the horde amasses. If there is a good team at the time, then you can easily get a bronze medal and gain this achievement. Also as ChiefMon and visheyry said, if you are incapped well trying to get this achievement, NEVER FIRE YOUR PISTOL(S) OR THIS ACHIEVEMENT IS VOID IF YOUR TEAM GETS A MEDAL!

I personally found it easy to do on the Burger King level of Swamp Fever. Just grab the axe on top of the building and keep mashing away. If a tank comes, keep avoiding it untill the others (preferably AI) kill it. If things really go bad, just use adrenaline and keep running around. Shouldn't take you more than a couple of tries.

We found it easier to do it on Hard Rain sub level Sugar Mill. At the back of the map, ground level there is a little room with only one entrance. Sounds dodgy but it funnels the zombies right into your bullets. There's also a bench that can be used to jump onto the lockers avoiding 90% of the hits you'd get on ground level. When a tank comes just pummel him, three ppl will get him down quickly enough. This cheevo really down to luck. Depends how fast the specials come at you. Keep plugging away it'll pop

I had earned the bonze before and needed the silver to get this achievement. I deleted the cache and refused the update and did the beat the rush glitch on The Parish and it popped for me at 4:00...thank god! YOU DO NOT HAVE TO 7:00!