Quite an easy achievement, best done in a group of 3 - 5 gears but can be done on your own or in split screen.
You must complete waves 1 - 10 on Highway one of the new dark corners maps.
The first 7/8 waves are fairly simple with the chance for you to stock up on ammo on the big power wepons. The last couple of waves are potetionaly a pain, the bloodmounts on wave 10 and the maulers swinging at you soon knock you out so dont be in a position where you are trapped e.g spawn or where the flamer/boomshot spawns.
Frag grenades, Boomshot, Torque Bow are best for damage to big bad guys, whilst if you have got enough people, have somebody fire the mortar and the others cover him. The mortar can take out 5/6 grinders/maulers in one shot if fired well.
You must also be level 10 in mulitplayer, this requires 13,576 exp earned in multiplayer matches, easily followed by a box poping up at the end of each match in the bottom right. Also next to your name tells you the rank you are currently at.
Once you finish wave 10 or reach level 10 in mulitiplayer (whichever comes last) the achievement should pop up.