This game has a memory. And what I mean by that is it remembers everything you make, if you die, or reload your game.
I waited till after the "Shrouded Bunker" mission, and I collected the recipe, and enough RC parts to have 15 RC's on standby ready to build.
Next, make a save, or just save your game like normal. Then go to the engineer section of the back button and find the RC, or whatever you have the most of; then click "Build All." It will build all of them one right after another. If you don't have a lot of one thing just build everything in your inventory you are able to.
After you built all you can, reload your game, and do it all over again. After you have hit 50 things engineered the achievement will pop.
If you have any issues with this guide, please comment.