Doom says a lot of stuff here.
I got Bridge Over Trebled Slaughter with one other human on easy. When the tank spawns (it'll happen as you are passing an 18-wheeler type truck with an ammo dump) you can just run past it. If you do this, there won't be a 2nd tank (this is my experience, at least). If someone gets incapped, leave them. Everyone gets it as long as the finale ends in less than three minutes. This finale doesn't require much teamwork anyway, as special infected don't happen very often. You should try to stay together just in case, but under no circumstances should you turn around to help someone.
If you're trying to do Wing and a Prayer on the wing of the airplane, then you're insane. Stay inside the passenger area after opening the door. There's a seating aisle that you can get into and just crouch down. Have someone that already has the achievement (Hello) standing in front of you with a melee weapon. While you're doing this, make yourself useful by shooting out the window behind you to get infected coming in on the wing.
I can help with Beat the Rush, but you pretty much have that one down- take turns. The important thing is not to get incapped, since you'll probably accidentally fire your magical pistol as you go down.
I know how ironic it is that
I'm giving
you advice here, but I have all these achievements and these methods are what worked for me. I'd be happy to help get any of them (and any others) as long as I'm not, like, doing schoolwork (so, not tonight).
On a related note, I'd be glad to help anyone who needs to get Guardin' Gnome. I can carry it around since that'd be more useful than me trying to help fight. Find me on steam when I'm not busy and we'll set this up.
Also, I don't know how any of you use the sniper rifles ever. Auto-Shotgun in L4D1 (although I still go M16 because it's more satisfying to spray bullets everywhere), FN SCAR (I don't know what the in-game name of it is) in L4D2. The SCAR (aka burst rifle) is accurate enough to pull off 100% headshots at non-insane (and yet safe) distances while running, and is my favorite weapon to use across The Bridge. I've never used the Magnum, because when the horde comes I want something that is going to either be fast or kill a lot of things at once. Dual pistols are fast, and the melee weapons are both.
