Any Hero completed the main quest without exploring more than 20 % of the map.
Rather simple. I recommend not doing a single side quest until the main quest is finished on a bronze campaign. This way, you do not do any unnecessary exploring. Look at the map, find the gold ring, and go straight to it, taking any shortcuts possible. Also, make sure you do not equip any items that increase visibility range, as it can explore more unneeded areas. By the end of my campaign, I had around 17.1% explored. And, this was with a few screw ups with getting lost and exploring extra areas.
Note: "Cartographer" achievements may pop at least .5% before they are supposed to. This possibly has a negative affect on the outcome of receiving this achievement if you explore within the 19.5% to 20% range.