That's 5 different gamertags within 30 seconds. I hesitate to put up a solution for this as I got this achievement when I was least expecting it. To me, this originally looked like the toughest achievement in the game to nab.
First, I'll offer some advice that I think will help for those going into a match specifically for this achievement, then I'll share how I got it.
1) The more the merrier - the achievement requires 10 gamertags in the match, so at least 5 vs. 5, but obviously, the more people in the match, the more likely you'll be able to eliminate 5 DIFFERENT opponents.
2) Riflemen and Marksmen should play conservatively - use semi auto fire (single shot) and crouch or lay prone for steadier shots. Try to set up shop with a good view of a lane of opponent traffic where they might not expect you to be. Running and gunning will just get you killed.
3) Automatic Riflemen, I think, could play more aggressively, using their high ammo capacity as an advantage. See if you can flank the enemy position and rush them, gun blazing from the side or behind. You may get them to panic and easily mow them down.
*3.5) Sounds like this must be done with bullets and not explosives, so well placed grenades and C4 are unfortunately not an option.
4) Have patience. This achievement may have more to do with luck than with skill and 30 seconds can be quite a long time in a GRAW 2 MP match.
Now, for my lucky snagging of this achievement:
I was finishing up my "Sniper Expert" achievement in a Team Elimination match on the Outpost map. I was lying prone in a corner of the map (southwest, I believe...lots of trees) with a view of a lane of traffic the opposing team consistently ran through as they came from the direction of their spawn point. Using an M-107 rifle (the one-shot beast), I was picking off anyone who slowed down enough to give me a clean shot, or who advanced toward my corner. At one point, after I downed two opponents in quick succession the achievement popped.