Because the final skill captures all cities without giving you the option to change the city type, if you plan to use it you need to make sure you already have 8 military cities or that enough of the un-captured cities are military or else you won't be able to get this achievement.
If you prefer an trick that makes sense in English. Make sure that at the start of Civilization Stage you capture every single spice geyser except the ones you can't reach and the one that is closest to your city, if you capture that one, other cities will rise up and steal the rest of the geysers. Finally capture the first geyser and just let the cities rise up. Spot all the religious and economic cities and target them first. When it comes down to just you and one other city you want to make sure it's a military city that way if they decided to "band together" with you you'll have every city as a military one.