This mission requires you to gather 8000 minerals with your SCVs and Reapers. Your SCVs will automatically start harvesting but pay attention; since the minerals in this level are all located in low areas they're subject to lava flooding. This means you'll have to clear out all your SCVs periodically so they don’t melt, but don’t worry, your adjutant will warn you when the lava is rising and there's a timer as well.
1. Don't build
The trick with this level is trying to build as few units as possible since every mineral spent prevents you from getting closer to your 8000 goal. After the first flood Tosh will give you some Reapers to use. Reapers are extremely fast and powerful, though they can’t take much damage. This will be your main squad, and if used correctly will be the only assault units you'll need. Very light Zerg attacks on your bases are likely so it doesn't hurt to build a 2-3 marines and a medic to protect it as you're exploring with the Reapers. The medic can also heal your Reapers of any damage they've taken, making them doubly useful.
2. Free the Northern base
Immediately to the north is another Command Center and some additional SCVs, though you'll have to kill a few burrowed Zerg to get to them. Have the Reapers kill the Hydralisk first and then the Zerglings. The Reapers are fast enough to shoot the Zerglings, retreat back a little bit, and shoot again without the Zerglings ever catching up, this will allow them to handle almost any melee unit with no risk as long as you move them properly.
Once you've got the additional units, land the command center as close to the nearby minerals as possible to minimize the time it takes for the SCV's to deposit their load, just make sure to allow enough time for the command center to get out of the lava zone. Directly to the northeast of the base and SCVs you just got are some additional Reapers, though you'll have to kill a few more Zerg to get them.
3. Search for mineral pick ups
As your SCV's are gathering minerals send the Reapers around the map looking for collectible mineral pick-ups, of which there are a lot. The Zerg base is confined mostly to the South and South East side of the map, so avoid those areas; everywhere else is fair game. Make sure to use the Reaper's leaping ability to check high areas as they almost all have large collections of minerals.
4. (Achievement) Kill the Brutalisk
The bottom left of the map contains the Brutalisk, your secondary mission objective. This huge Zerg creature will only attack if provoked but will one hit kill any of your units it manages to hit. Fortunately your Reapers are much too fast for it and can easily defeat him using the same, run back and shoot tactics used for the Zerglings. The Brutalisk will only follow you for so far before returning to his area, though you can easily shoot him on his way back and draw him out again. There's an achievement for killing him by drawing him out into the lava, and this is easily done with a Reaper, just draw him out as far as he'll go with a few seconds left and have the Reaper jump up to safety when the lava arrives.
5. Rescue the last of the Reapers
There is a large cache of minerals and the last of Tosh's Reapers at the bottom of the screen but be careful as this area is quite literally in the midst of a Zerg outpost. Be quick about it if you want to minimize casualties, or rush in if you're close to the goal and collect these minerals to push you to 8000.