This is unlocked by purchasing the corresponding skill in your skill tree (Tier 1). Each node of the tree has a cost of 1 skill point (see picture below)
When you hover over each node in your skill tree, a description of the skill linked to that node will open. You'll see that there are various types of skills associated to the nodes. Some nodes unlock additional slots in your backpack, some nodes unlock survivor slots for your survivor squads, some other nodes unlock defender slots for your defender squads, etc. etc. etc.
The size and shape of the node indicates the type of unlocked skill. So, for instance, sqaure nodes are linked to unlocking squad slots, small round nodes are linked to personal or team stats (backpack size, health, tech, etc). Hero unlocks are the big circles (but not all of those big circles are hero unlocks).
In this case, you are looking for the large circle that is linked to the Outlander Hero. Once you purchase that node for 1 skill point, you'll get the achievement.
1) To be able to purchase the Outlander node, you'll have to purchase all previous nodes that lead up to that one. That means you'll need several skill points to get it.
2) This achievement was initially bugged for anyone who purchased the limited edition (or whatever it's called) that comes with an Outlander hero and a Ninja hero. Thanks to Darth Maul FTS for confirming that this has now been fixed. If you had this issue, you should unlock the achievement right after logging into the game (after having updated it, of course)