For this achievement you need to explore all 100 rooms in a normal run (5 levels).
According to the devs,you'll have 2 secrets in a realm (level) if they have only 1 shop (instead of 2).For me it was level 2.
Be careful,you can have more than one realm with 2 secret rooms !
Also, I don't know if you can see some details for the secret walls. I just know that some light/dust pass through them in level 1.For the other levels I blasted every wall haha.
So simply blast every corner of every rooms where there's less than 4 exits (only your gun can destroy secret walls).So use wisely your ammo !
You can also try to buy a better gun,it's recommended for this.
If you have the map of the level, you're good for this one.
Good luck and have fun !