Your Outposts are the 2 guns in the back next to your pop. In order to unlock the achievement you need to win the match without either of your outposts being blown up. They can take damage they just can't be destroyed.
If you want to ensure you keep your outposts intact, try keeping a fortification ready to use just in case a enemy makes it to them. This will draw attention away from the outpost and allow the guns to attack the enemy.
As mentioned in the comments and I also can verify, Horde mode does count towards the achievement progress also. I recommend the coin hoard, it is only 2 waves but extremely easy even for new players that don't have a lot of high leveled pins.
Win the match by any means (You don't have to get all 3 points) with both Outposts/Turrets still standing and you will increase your progress bar which you can check at any time by going to the MISSIONS tab and clicking/tapping on ACHIEVEMENTS which is located next to BOUNTIES