Shield World (level)/Walkthrough
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Quickly set up your base, get your Tech Level to at least 3 by building a double reactor and occupying the bonus reactor north of your base. After that, start building Scorpion tanks. Don't send in infantry, since the almost-omnipresent Flood launchers in the level will turn your infantry against you with spore clouds. This means you can have one extra building space since you leave out the barracks.
Research the Canister Shell ability. This level also has several aerial Flood units, so it doesn't hurt to back the Scorpions up with several Wolverines (who annihilate anything aerial). It may also help with their volley ability, which does significant damage to ground units.
Clean the area for Alpha team first, which can be done with a fair amount of Scorpions. If you find some Flood Launchers attacking your units and they are too far away from your units, freeze them with the cryo bomb. Make sure your units are not in the blast radius.
Gremlins and Pylons
A Pylon will soon show up next to your base - have a Gremlin stand next to it to increase the time on your timer with a few minutes. You must do this at least once or you'll find yourself having very little time to complete this mission. Since you don't need to buy the Gremlins yourself, just clear the paths to the pylons and let the free Gremlins do their part.
There is another Pylon just north (right) from Alpha Team's position, another Pylon can be found between approximately your base and Charlie's location, and the last is roughly between Bravo and Charlie.
Helping Bravo Team is more difficult, and will require a larger squad of Scorpions and support from Wolverines. When you finally reach Bravo Team, don't miss the 10th Black Box right at their pinned down location. If you want to collect the Skull in this level, use a Hornet to locate it (after killing 350 Swarms) northeast from your base. There will be several Flood Eggs there.
When you get to Charlie, manual assistance is mandatory due to the transporter being canceled because of the dangerous circumstances. Have a few Scorpions guide the whole Charlie Team back to the base and make sure you have several Scorpions back at the base as well, which will soon be attacked.
When all teams are evacuated, you have succeeded.