Defense Grid: The Awakening

I followed Socket Pop's strategy (as I understood it) to get the achievement myself, and am just uploading the two video guides I made below as visual aids to accompany his description.

The key to unlocking gold medals in this game is finishing the map with ALL of your cores and a considerable amount of money in the bank. The money is the key. If you go out buying towers and upgrades that you don't need, the best you'll ever get is a silver medal. You can really only build the absolute bare minimum number of towers necessary to protect all of your cores if you want gold.

An important thing to remember is that this game pays you interest based on what's in your bank. The more money you have in the bank, the greater your interest payments. This is why it's important to delay your tower/upgrade purchases until you really need them. Buying towers that you don't need will just cut into your interest and making getting gold that much more difficult. Getting the timing down on what to build and what to upgrade when can be tricky, so if you notice some aliens sneaking through your defenses, don't hesitate to use the BACK button to go back to your last checkpoint and adjust your timing.

All modes of Overflow can be beaten using this strategy.

Block the east entrance to the reactor with two Meteor Towers.

All aliens will cross the entry bridge, then naturally turn south to reach the reactor. No guiding towers are necessary. Your kill box is the center section just south of the bridge.

Build a horizontal row of Gun Towers across the kill box, which forces the aliens to zig zag across all sections south of the bridge and enter the reactor from the west.

You'll eventually need two fully upgraded missile towers to deal with the flyers. You can put them pretty much anywhere along the flight path, and upgrade them when you can afford it, as the waves progress. I like to put one just north of the bridge, blocking the center north entrance to the kill box. The other I put by itself in the section just south of the alien entrance.

In the section south of the kill box, place three towers near the entrance to the reactor, which funnels the aliens past the row of guns in the kill box for a second pass. I like to use lasers here, so that they are on fire while circling the reactor and taking meteor hits.

Once the route is established, put a Temporal and a Command in the kill box. I like to add a concussion and a couple of lasers or incendiaries where they fit and keep the aliens on fire. A laser on the north edge of the kill box can hit aliens as they're crossing the bridge, too.

Upgrade everything to level three and this arrangement will kill everything that comes through it, up until the final three shielded bosses.

After the last flyers are shot down in wave 25, sell the missile towers and start earning interest. I let the bosses march back through my kill box, then place cheap gun towers in their path to force them to backtrack through the kill box again.

Once you're down to the last boss, you can sell all your towers and earn interest while he's marching to the exit. Then, orbital laser him when he nears the exit.

In the 40k limit mode, this arrangement can be purchased and fully upgraded with about 17 or 18k resources left over. (More frugal strategists can do better, I bet, but this was enough to get the gold medal.) Then, just fast forward until the bitter end.

This works for Campaign Reverse, too. They follow exactly the same route with no further urging, except they leave by the long bridge instead of entering that way.