For this achievement you have to accomplish finishing a map without a thief being deployed AND without losing a single crystal. This can be tough, but luckily there's a video guide for it! :)
Also this is kthili sands.
"Some tips for this cheevo as you have to be very accurate in what you place:
-After he has finished placing look at your money if you have more than him it's likely you have missed something out.
-Also it is hard to see what he is upgrading some of the time so get it full screen and look for the cursor or white range ring to see which one he is upgrading.
-If you can't see through the upgrade menu you can let it play until the end of the round and then pause it and there should be a small amount of time in which you can view the levels of his placements, soldiers will be out of position if they have been attacking so be careful with these.
Posted by Subliminal Cuts."