Hill 400 starts out with you and your squad mates moving up the hill and taking out bunkers. While there isn't an easy way to do this it helps if you make sure you use the scoped German rifle (after the first bunker you breach there is a second door you can blow open. It has ammo and a scoped rifle). This makes it easy to go prone and snipe.

When the mission first starts out you have to snipe the mortar teams as they come up the hill. There is no easy way to do this. What's nice is that you can take your time (unlike silo, where you have to quickly take out the mortar teams before they blow you out of the tower). Do the best you can and stick around the perimeter. Make sure you run to the fox holes and let your squad clear the area before you move on.

When you're holding out for reinforcements, after you snipe the mortar teams, you'll have to take cover from artillery fire. Make sure you run for cover in the middle of the hill in the bunker. When the artillery fire is done head out of the bunker and make a left, hugging the bunker wall. You'll see some sand bags and a M240 mounted. Follow the bunker wall and make another left. Now you'll see another sandbag with a mortar behind it. Run behind it and go prone (your feet to the sand bags and facing the bunker wall). Your squad mates will hold up on your left and you need to watch your right. You might encounter 4 or 5 guys while the whole barrage is going on. Just sit tight until you have to run from artillery fire. Do the same thing, hold out in the bunker and run back to the same sand bags. During the second wave I didn't see a single German. There was a tank blasting me but the sand bags kept me from getting hurt. You might see one or two guys, but just hold out till the timer runs out, then BAM achievement.