This achievement can be extremely irritating, depending on how many locked doors you have in your playthrough. For example, I explored every available room in one of my play throughs, however I had something insane like 10 locked doors and only had 3 keys given to me throughout the game. Make sure you are well stocked on food and craft a great weapon early on to make the exploration easier and quicker, and then pray to the RNG gods for minimal locked doors and abundances of keys.
As well, once you complete the game the first time around, you will unlock a perk that allows you to unlock doors without a key. It's well worth it to equip this perk if you plan on farming this achievement. However, you can only use this perk once every 100 turns, so make it count.
NOTE: You do NOT have to kill the monsters in the rooms for them to be counted as explored.