"This is a mountainous terrain rich in resources for establishing a prosperous colony. This region is named because of a hollow to the Northwest that is plagued by Harpies. This lethal type of infected is attracted to the place that is believed to be an old common grave judging by the bones that appear on the ground. Harpies seem harmless as they are usually seen crawling very slowly on the ground, but when they feel the presence of prey, they throw themselves at it with such speed that it is impossible to flee from their sharp claws. Establish a large colony and obliterate, once and for all, the dreaded Harpy Nest."
This may well be your first encounter with the Harpy, depending on what order you're doing the maps in. While the Chubby is only a focus-fire target in that he's unlikely to go down without being focused on, Harpies are "drop everything and kill this" targets, at least at this point. While not THAT tough, having only half again the health of an Executive, they are extremely fast and very, very deadly, capable of running down a Ranger and killing them in two hits. They can also leap over walls, meaning they can bypass most static defenses and wreak havoc on towers or buildings behind them. Harpies are also very, very sensitive to Noise, being aggroed by combat noise at far greater distances than any other Infected you can expect to regularly meet.
As the game highlights by showing you their nest, this map has an ENORMOUS number of Harpies in the northeast who, if carelessly aggroed, can easily overwhelm just about any military force. The good news, however, is that the map is very blatant about showing you where the "danger zone" truly begins, and as such the odds of blundering into this deathtrap are small.
Firstly, clean out and fortify the area around your Command Center. Be wary about building too much on the map's limited farmland -- you do have a quite high population target to hit and you'll want all the farmland you can get. You will also have to deal with Swarms from both East and West, and later on the North (luckily there will only be 3 common swarms).
Focus first on moving West -- there's a Tavern of Doom on one of the eastern tracks that will take longer to clear, and the iron is on the West side. Throw up defenses at the split path by the east end of the lake, and then clean out the West. This will run you straight into the narrow passageway to the Danger Zone, which is both unmistakable and outright labeled with a sign on the front, so just wall it off and ignore it for now.
Once you're done cleaning up the West, then go and tackle the rest of the East. This is easier said than done, as the Infected include a fair number of Executives and even Chubbies hanging around at this point, but if you've made it this far, you can handle them. Be thorough to make sure no lone Infected are lurking in the shadows of the trees or cliffs, or slip around one side while you're focusing on the other. Make sure you have appropriate swarm defenses on the exits, you don't want to have to be micromanaging them once you start working on the Nest.
With the south side secured, proceed North past the Danger Zone chokepoint and up the final exit canyon at a cautious pace. You may start to see lone Harpies coming out of the Nest at you if you're making enough of a racket, but that should be a rare problem at this distance. Clear out the North canyon and secure it with defenses, then head back to the crossroads and prepare to take on the Nest.
Harpies are best fought with Snipers behind a line of Soldiers, unless you've really been focusing on your Soldiers' technology to this point, in which case you may be able to get away with just Soldiers. Even if they're not pumped, however, Soldiers can take Harpies to the face far better than Snipers with their significant damage reduction armor, so having a line of them in front of your Snipers is important. Either way, however, advance CAUTIOUSLY up the final canyon, since it does also feature another Village of Doom. If you have enough time left in the scenario to be cautious, walking defenses up the canyon with your soldiers can be a good idea -- Harpies can leap over Walls and such but will still be slowed down by them, Towers will still protect your units, and any chaff will be held up.
It's after that Village that you can really expect to start pulling Harpies en masse. Keep an eye on the Nest as you approach. As soon as you see Harpies start to pull out of the mass in the Nest in any number larger than ones and twos, STOP YOUR ADVANCE and hold in place -- the noise you make will only grow as you fight whatever's in the way and the first Harpies, which will inevitably draw more. Hold your ground and mow them down as they come. Only once you stop pulling new Harpies from the pile should you resume creeping forward, and then repeat the process. Eventually you'll have thinned out the horde, and then it's just a question of finishing the mopping-up and making sure you've hit the population quota.
Push with lots of towers is highly recommended. However, if your army get overwhelmed, you need to destroy tesla towers between your base and the frontline, or harpies will come straight at your base.