You will finally be able to build some new units in this mission - scouts! While their ground attack isn't great, the anti-air is excellent. They are, however, quite expensive so you shouldn't waste them needlessly. You should proceed to build up your base with 12 drones on minerals and 4 on gas. Keep your forces ready to repel zerg attacks, as they will certainly come and in more numbers than before. You can use your scouts to well, scout out the map some. Move in a big circle moving outward from your base to reveal some of the map. If anything attacks them, run! They should be available to help protect your base in its infancy, and destroyed scouts can't do that at all.

Of the new upgrades you can get are the possibilities to research the faster movement for zealots (at the citadel of adun) and upgrading your scouts. Pick air attacks (cybernetics core) and plasma shields (forge). Your dragoons should get their increased range upgrade, of course. Expand your base some by placing pylons out to the west. Make about four of them in a line stretching across the border and put a photon cannon on each. This will prevent the Zerg from entering your base unapposed. Your scouts you started with will do fine for repelling air attacks throughout the mission, so you should leave them and the other units you started with home to defend. A photon cannon or three north of your base, well spread out will help to slow down any attacking mutalisks from the north, also.

Now if you use your scouts to look to the west of your base, you'll see one of the Zerg colonies. It's quite large. You'll see a huge skeleton of some creature (quite cool) and can scout out the borders of it. You'll see the location you have to assault from, which is a ramp to the northern side of that base. The other zerg base is north of yours, take a peek but don' get your forces hurt - this will be the first base you attack.So, with that new knowledge you should prepare for a ground assault to cut back on the Zerg attacks.

Make sure you have the upgrades I mentioned earlier, or have at least started researching them, then begin warping in the units you'll need to take down the Zerg colony. Sixteen zealots, ten dragoons and four scouts should do just fine. If any upgrades complete while they're all warping in, research more. Ground attacks and air armor would be helpful. Keep each unit type in their own group and proceed toward the northern Zerg colony.

When you get here, you'll see many scourge in the air. That makes it unsafe for your scouts, so shoot those down as you see them (don't trouble yourself to accomplish this) so your scouts can do anti-air. Your zealots should focus on taking down enemy units while the dragoons help them, occasionally focus-firing on a defensive colony. This base is much easier to assault than the second, and is a great place for an expansion. Build a nexus and assimilator there (you will have to wait on the creep to erode), and move your probes (who are probably out of minerals) up to harvest once they're done. Leave your original probes on gas though, since they can get some resources even if it's depleted.

Now, leave some units there to defend as well as some photon cannons and pylons, well spread out. The final base is much harder and you're going to need a lot more zealots to accomplish it. Get up a total of 24 zealots (some of your leftovers from the first assault can go toward this) 12 dragoons and six scouts. This will be plenty to romp the last Zerg base. Do just like before, but this time you probably won't have to worry about scourge. Again, don't let your scouts take needless damage since they're not very useful in ground combat anyway, use them for anti-air and only that, until the way is clear for them. Deal as much damage as possible, and if you fail you can send in more of whatever seems appropriate to you (probably zealots and dragoons) to finish the job.