I got this Achievement on the first Zone by starting from Area One. It popped right after the scoreboard came up. I was playing alone and I skipped the Underground Area. Even so, I got this Achievement at the end of the level as well:
When you finish a Zone, you will be picked up by a dropship at the end of a cinematic and it will say "Zone Complete" then a scoreboard pops up displaying your stats for that zone. The first Zone consists of four Areas. You can skip the Underground area and still get this achievement.
You shouldn't have much trouble doing this at all on Easy but here's a few tips anyway:
-Use your Grenades (RT) often, especially against heavy armor and bosses. You start with 10 and get 10 more at the very beginning of Area One.
-Save your Nukes for either the Spider Boss or the giant wall at the end of the Zone.
-I find the Flamethrower to be the most effective weapon after you upgrade it at least once. But instead of holding the stick, just keep rapidly tapping it towards your targets. This will make it spray several small embers that actually guide themselves to the target. This method works great because you don't have to be accurate with your shots and they do more damage than the Chaingun. This weapon is very effective throughout the entire game.
I hope this helps. If there's anything I can do to make this solution better, please leave a comment. Thanks!