Left 4 Dead

In Versus mode, when you are going through the hospital trying to get the elevator, find an oxygen tank. Carry it with you until you get into the elevator. Once you are in the elevator, choose one of your teammates and throw the oxygen tank at them. They will fall through the elevator floor and die.

Note: This is an extremely difficult trick to complete. Once everybody is in the elevator, order one person to get outside of it. To do this correctly, the person that is going outside must be slightly in front of the blood spot of the elevator, so that the game thinks that it is safe to close the doors. Immediately start the elevator the moment the person starts walking out. If the timing is perfect, it should work. Note: This was done while everyone on the server had a two bar connection, making it not lagged but causing events in the game to slow down at parts.

When the survivors get to the area where they have the minigun and the gate they have to open up, there will be a hole in the floor which you can spawn as a Smoker. You can pull them through the floor countless times.

Complete the following achievements to unlock Xbox Live Gamerscore points.
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Helping Hand (20 points): Revive 50 incapacitated Survivors.

Field Medic (20 points): Heal 25 Survivors with a first aid kit.

Pharm-assist (20 points): Give pain pills to 10 Survivors.

My Bodyguard (15 points): Protect any Survivor from an attacking Infected 50 times.

Dead Stop (10 points): Punch a Hunter as he is pouncing.

Cr0wnd (30 points): Kill a Witch with a single headshot.

Untouchables (30 points): No Survivors take damage after contacting the rescue vehicle.

Drag and Drop (20 points): Rescue a Survivor from a Smoker's tongue before he takes damage.

Blind Luck (20 points): You or another Survivor take no damage after being vomited on by a Boomer.

Akimbo Assassin (30 points): Survive an entire campaign using only pistols.

Hero Closet (10 points): Rescue a Survivor trapped in a closet.

Hunter Punter (10 points): Shove a Hunter off of a pinned and helpless Survivor.

Tongue Twister (20 points): Free yourself from a Smoker who has grabbed you with his tongue.

No Smoking Section (15 points): Kill 10 Smokers as they are pulling helpless Survivors.

101 Cremations (20 points): Set 101 Infected on fire.

Do Not Disturb (20 points): Sneak past all Witches in a campaign without disturbing one.

Man vs Tank (30 points): Single-handedly kill a Tank.

Tankbusters (20 points): Kill a Tank without it dealing any damage to a Survivor.

Safety First (30 points): Play an entire campaign with no Survivors taking friendly fire damage.

No-one Left Behind (20 points): Beat a campaign with all 4 Survivors.

Unbreakable (30 points): Finish a campaign without ever being healed.

Witch Hunter (20 points): Kill a Witch without any Survivor taking damage from her.

Red Mist (20 points): Kill 1000 Infected with a mounted machine gun.

Pyrotechnician (20 points): Blow up 20 Infected in a single explosion.

Zombie Genocidest (20 points): Kill 53,595 Infected.

Dead Giveaway (10 points): Heal a fellow Survivor when your own health is below 10.

Stand Tall (20 points): Survive a campaign without being incapacitated.

Zombicidal Maniac (30 points): Survive any campaign on Expert.

What Are You Trying to Prove? (35 points): Survive all campaigns on Expert.

Nothing Special (30 points): Survive a campaign with no Survivors taking damage from Special Infected.

Burn the Witch (10 points): Light a Witch with a Molotov.

Towering Inferno (10 points): Light a Tank with a Molotov.

Spinal Tap (10 points): Kill an Infected with a single blow from behind.

Stomach Upset (20 points): All Survivors complete a campaign without being vomited on.

Brain Salad (15 points): Make 100 headshot kills.

Jump Shot (20 points): Headshot a Hunter while he's leaping.

Mercy Killer (15 points): Survive the No Mercy campaign.

Back 2 Help (20 points): Leave a safe room to save an incapped teammate and bring them back safely.

Toll Collector (15 points): Survive the Death Toll campaign.

Dead Baron (15 points): Survive the Dead Air campaign.

Grim Reaper (15 points): Survive the Blood Harvest campaign.

Ground Cover (30 points): Save another Survivor from a Special Infected while on the ground.

Clean Kill (10 points): Shove a Boomer and then kill him without him splashing on anyone.

Big Drag (20 points): Drag a Survivor 100 feet with your tongue.

Chain Smoker (20 points): Constrict two Survivors in one life as a Smoker.

Barf Bagged (20 points): Cover four Survivors with Boomer bile at once.

Double Jump (20 points): Pounce two different Survivors in one life as a Hunter.

All 4 Dead (30 points): Kill all four Survivors in one life while playing as a Tank.

Dead Wreckening (20 points): Dole out 5000 total Survivor damage as a Special Infected.

Lamb 2 Slaughter (20 points): As an Infected, incap a Survivor who has entered and left a safe room.

Complete the following achievements to unlock Xbox Live Gamerscore points.

Complete each achievement to receive the allotted gamerscore.