Playing the campaigns you should pass 4000 easy, so this is the most efficient way of gaining this achievement. However, if you want to get this before playing the campaign, go to skirmish (under the Single Player menu) and load up a big map such as Seton's' Clutch vs. the normal AI. Set the unit cap to 500, Pre-built base and 20 minute no rush option. Start out by building up your economy then make 10 - 20 land factories and start pumping out ALL SCOUT UNITS. With this method you will reach the 500 unit cap before time runs out many times over. When you reach the 500 unit cap select all of the scouts by zooming out and double tapping on any one of the scouts. Then press then navigate to the Self Destruct button in the lower right corner. Keep building until you reach unit cap 2 more times then save and then the achievement will unlock after saving.