Here is a link to a youtube video of me obtaining the pack-a-punch before the end of round 6, solo. Sorry it's kind of long, it's the full game and doesn't have any commentary, so it's a little boring. It's like a video equivalent of a "wall of text". :D
Credit for the train/loop always goes to the youtuber Syndicate - http://www.youtube.com/user/TheSyndicateProject
Here are some pointers:
1) Don't get quick revive or juggy. In addition to saving money, you won't waste a round on monkeys.
2) Starting in round 3, run a train/loop wherever you prefer and go for collaterals. I ran mine by the PHD Flopper lander, just running around from corner to corner.
3) Don't hit the box. Depending on where you want to run your train, get either the MPL or MP5K, that way you can run for ammo if you need to.
4) If you get Max Ammo, and you should at least once in the 7 rounds, reload your M9 and as you are forming your train behind you, turn around and empty a clip through all of them. It won't do any damage and you'll get a ton of points.
5) If you get a Nuke, try and pop it off between rounds where it won't kill anyone, but you'll still get 400 points.
6) Don't bother picking up other power-ups like the death machine or insta-kill. You need the points.
You have to get it while round 7 is still active (round 7 at the latest, I got mine in round 6), so you want crawlers. And remember that calling the lander near a zombie will kill it. Go do all three landers and pack your gun without killing your crawler(s). Achievement unlocked.