It's a trap achievement

First things first, you will need to have Junta Military DLC because this DLC has The New Sparta mission and in this mission there will be many rebels attacking you.

Play the mission as you would play any other mission and build up your economy so you can afford to buy Mausoleum and upgrade it.

Once you get to the point where main quest requests you to HAVE 30 soldiers do a hard save. Because you might fail. Hopefully you have built Mausoleum and upgraded it by this time. Keep in mind though build it somewhere in empty spot because if there will be any civilian casualties then achievement will not be unlocked! (From my experience)

Also! There will be couple scripted rebel attacks which will ignore Mausoleum completely, do not worry about it.

Anyway, after you recruit 30 soldiers there will be scripted rebel attack which is completely fine and they most likely will target one of your bunkers. BUT! Instead of repelling their attack what you do is this DISMISS all soldiers and close up every free spot in every military building even in your own palace, let them win their attack and blow up on of your own buildings.

Now, what you want to do next is wait about for a month and issue "Lure the rebels" edict so everyone who survived before will attack your Mausoleum now. There should be around 20 guys.

If you fail reload your save and try this strategy again! I did it with second attempt because first one had 1 civilian casualty.

If i forgot something to mention please let me know.