When using the M4A1 I personaally recommend using kick,red dot sight, and a silencer the silencer is for sneacking around the battlefield without being detected when firing and the red dot sight is good because it's not very big so you can see your enemies very clearly before zooming in to finish them and kick is to reduce your kick when firing the M4A1 it comes in very handy when playing online.
Go to the second chapter and select easy and use 1 hit kills with yoour b.a.r. For the assault rifle achievement.
Go to the forest chapter and select easy and use your bolt action rifles(kar98k, lee enfield, can't remember anything else) to beat it for the rifle thang achievement
Just throw you own grenades on the ground and pick them up and throw them away for the grenade achievement.
Sit back and wait for your guys to kill everybody and only use your bullets for guys that are immediately dangerous to you for the conservationalist and allergic to bullets achievements, also it takes alot of patience for these 2 but do it on the 2nd lvl and it's a little quicker.
If your on a hard mode or just having trouble with chambois, you can look for boxes or 55 gal drums that are sprawled out everywhere, if you take out you're thompson and shoot them they will move fast but lay down on the ground and shoot at it and stay behind it for portable cover, thats the only way I found for myself to beat chambois on vet
If you are in a situation were you have to kill a group of germans (Something like a bunker or a room) you should cock of a grenade by holding the RB button instead of tapping it this will reduce the time befor explosion.You should release it when the red metre reaches 1-2 depending on the distance of the target it will then explode strait away.This is usefull because the Germans can pick the grenade up and throw it back! *This is very useful