Left 4 Dead achievement questions

So I have been playing a lot of left 4 dead lately trying to get most of the achievements. My question pertains to beating all levels on Expert. I have beat all four on expert now and for some reason it didn't give me credit for two of them. Yesterday I beat No Mercy and Dead Air but when I checked my stats it only registered me beating the other two levels.

Do you have to start from the very fist level and play it all the way through to get it or can you jump in at any point in a game and still get credit? I was pretty sure the I jumped in on the other two that gave me credit for beating, but not on these last two levels.

If anyone knows what is going on or had similar troubles please share your wisdom with me so I can put this to rest once and for all.

Ypu have to do them begging to end, although theres is a dashboard glitch to get them and you only ave to play the last level, tho i think u should play them all the way thru, its a lot funner ^^

Im sorry, but how in God's name is anybody supposed to get the "Untouchables" achievement? (It's the achievement where no survivor takes damage after contacting a rescue vehicle)masterchef56

You do Blood Harvest and you get on a rock at the very edge of the level. You have one person with a rifle sniping smokers and then everyone else have shotguns shooting infected that come out of the woods. One of my friends had me do it with him a while back, we got it in one try. You just have to have 4 peopleto do it without a doubt.

the jesus room in no mercy

Im sorry, but how in God's name is anybody supposed to get the "Untouchables" achievement? (It's the achievement where no survivor takes damage after contacting a rescue vehicle)masterchef56

You can either use the Jesus room which is in No Mercy or do what I did:

Play Death Toll and when you get to the house (Finale) have 2 people grab pipe bombs, and the other two Molotovs. Have three people head over to the docks (where the boat will come) and one person at the house. When the 3 people get to the docks call for the boat and run to the docks where your friends are. Hold off all of the zombies and when a Tank comes get one of the Molotov people to throw it at him. When the boat finally does arrive, get one of your pipe bomb people to throw it to get rid of the zombies.

I did this on easy and it, surprisingly worked quite well. Just stay at the docks and you should be fine.

you have to play each campaign without stopp. if you stop at one chapter, then start the campaign again at the level you left from, you won't get it.orb_03_2006

yeah beginning to end