Toy Soldiers: Cold War

Easiest way -

Casual, on the second level - Jungle Fever.

To get a platinum medal, you need to ensure that you get gold medals on all three sections.

a ) Don't let any enemies into your base for a gold medal
b ) As soon as you've killed the last enemy of wave (or only have a few easy ones left) hit X to start the next wave. On infantry only waves, I quite often launch as soon as it lets me.
c) Use as little money as possible.

For Jungle Fever, its actually quite easy. Follow the above tactics, and it can be done with only 3 turrets placed.

Place MG/MG/AT at the three turrets that cover the enemy spawns. Upgrade all three turrets to level 2, and then don't buy any more.

The best way to earn a plat medal is to play on casual.
In order to select casual go to the mission select screen for campaign and select the mission than press a then move the thumbstick to move to the casual tab and click a. You will default play normal if you select campaign and don't choose causual manually.

Now 3 factors go into getting the plat medal.

1. How many enemies breach your box. So don't let any get in.
2. How much time you save. So kill enemies fast and always use X to fast foward enemy waves.
3. Money saved (profit). So try to build as few turrets as needed to win to save cash. Also try to use turrets manually to maximize cash flow. And if you don't need to upgrade them than don't do that either.

If you obtain all 3 gold medals your final medal will be a plat medal.

Good luck

Edit: do not use any rewinds.