there are 15 levels total and this achievement should be completed with classic total victory(complete all levels without anyone getting through) on 1 you can only use one unit and trenches make sure you put the units in the trenches for added defence. one the second level you will get the gunner this is going to be your primary defence on levels 2-9 use these along with trenches to make choke points for the ememies make them go up and down through lines of gunners and replace them whilst upgrading them as needed. as you progress to level nine more techs will become available make sure you use these all to get the commander of technology achievement and they are verry helpful when used together. on level nine you get the ion cannon and suddenly the game gets easier IMOP. i used thes the same way by making choke points and used the occasional gunners in the trench if there were flying or fast units early on, but the ion cannon is fast, strong, and has a lot of hp making it a powerful weapon use it during levels 9-15 and youl be set!