Once you load up the mission Silent Cartographer you must finish the beginning (beach area) without a marine ally dying, following this guide you can do this in 5 minutes, first try.
Skulls: Bandanna (doesn't ruin achievement), Boom
Right when you start off, throw grenades ahead and not near marine, the area is extremely open and its actually easy to not hit marines. The only thing you have to worry about is throwing 2 grenades in the same area, 1 blowing the grenade back at you or your ally marines, since boom is on the radius of the explosion is increased. This takes seriously 2-5 minutes, take your time and don't rush too fast, just hit the enemies before your marines get up there.