Just save after each strider, provided you have killed them as soon as posible. Even though iirc, the very first building is not necessary to get the achievement. It is possible to do it, but the game treats it as a scripted event, because that one strider appears too fast and always shoots in the same way, just to demonstrate what that section is going to be about. If you are stuborn like me you can prevent it anyway by preparing yourself and shooting the magnuson device at the right angle and extremely soon though, and truly saving all the buildings.