Make sure you play this on easy.
There will be three waves of enemies that you need to defend your Humvees from. This is actually pretty easy if you prepare properly for it. All your Humvees need to make it through, none can be destroyed IN ANY SECTION.
First wave: There will be a bunch of enemies rushing down from the far left hand side when you start off. They are pretty simple, just pick them off one by one till the first tank appears. The tanks will take three Javelin rockets each to destroy them. Have you and your partner take it out as soon as you spot it. We did this wave with one person on the bridge and the other below.
Second wave: This wave one of you should grab a sniper rifle as your primary weapon. The enemies will start rushing from the left hand side of the bridge. Have the sniper watch the far right building as there will be RPGs (one or two) that will appear and try to level your Humvees. Have the sniper take them out while the other player takes out the other enemies on the ground. A tank will appear from the left hand side alley and you need to take it out before it clears that area. Both of you need to use your Javelins to take this out. You can try and put mines down, but we did not have much luck as the enemies seemed to take them out. However, others have had luck with this, so if you are struggling, toss some down.
Third Wave: You have to switch areas now. This will happen on the far right hand side. Grab some ammo and follow the Humvees heading over. Have one person still using the sniper rifle as there will be a few RPG guys and snipers in the building at the end of the area to take out. Once again you have to fight off a bunch of enemies. There will be one or two tanks that will show up this time ( We lucked out an only had one) that need to be taken out with Javelin rockets. This is the only real tricky part of the level since there will actively be enemies warming in faster and both snipers/RPGs to deal with. Once person should climb up on the building on the right hand side above the ammo crate and Humvees(there is also ammo on the roof). Here they will be able to snipe the RPGs and Snipers at the far end, plus watch for the tanks that roll in. The non-sniper should dispatch the rest of the ground enemies as best as possible.
The achievement will pop before you evacuate if you save all your Humvees.