This achievement is very easy and will come with natural progression. It is unmissable!
After the first few quests you will be rewarded with your first troops. Simply keep fighting to unlock new troops. If you want to speed this up, you can try to get some keys as a reward for quests or simply buy them with micro-transactions.
The keys can be used by pressing
on the world map (to open the menu) and going to the "chests" section. If you use the standard keys you will get 3 random rewards. This will speed these troops achievements up.
Just as a slight orientation for you guys. I got my 10th troop while I was level 4. So this should unlock rather soon after starting the game.
Hope this helps!
Any improvements? Please let me know in the comments below