How to obtain Hammers?

As far as I know there are only 2 possible answers to your question:

1.) By hammers you mean the icon you see below your nameplate when you are looking at the outside world, or inside your command center under buildings. This signifies you are at a specific rebuild. After you hit level 30 on your CC you can then start toward rebuild 1. Using spades and advanced blueprints to upgrade some of your buildings further. You need to do 10 constructions then the final "rebuild" to get to "hammer" 1. This goes all the way up to rebuild 3 so far. Many other buildings can also be rebuilt. If you click on a building and go to details, then more info, it will tell you if it can be upgraded and what benefits you get from doing so.

2.) You mean Demonic Hammer or some other final skill from an officer. You need to get an officer that has a final skill to at least rank 7 then but either full officers or learning books into their talent until they are all maxed out.

Hope this helps.