To level up your troops first press
when on the map screen. From there select "troops" and then select the troop you want to upgrade (You can get more troops by selecting Chests on the menu screen (requires keys) if you don't like any of your current ones). Leveling up your troops costs souls. These souls are going to be your limiting factor. The link at the bottom show how many souls it costs to level up each troop based on their rarity.
People are reporting getting one troop to level 9 and then upgrading a different troop to any level to unlock this achievement
1. This can save you more than 200 Souls and is not taken into account below. If, however, you have a task to level up a troop to level 10 then completing that task will reward you with more souls than it cost to level up to 10
2.
The lowest amount of souls getting to level 10 will cost you is
875 for a common troop while the highest is
1305 for a Legendary troop.
The fastest way to obtain souls, besides microtransactions, is by completing Challenges. You can access Challenges by selecting a kingdom on the world map and then choosing Challenges. You'll unlock more challenges as you complete Quests. Other ways to obtain souls are by defeating enemy troops in battle (1 soul/troop), using certain troops abilities, the Treasure Chest game mode in Zhul'Kari
3, using Glory in the Rewards section of the Shop (40 Souls for
10 25 Glory), and disenchanting troops (This deletes them
4). (I'll add more ways to earn souls as I find them. Let me know of any ways I'm missing.)
Required souls to upgrade troops:
http://gemsofwar.com/game-guide-troop-levelling/Thanks for reading, I hope this helps, and let me know of any improvements.
1 Thanks to RadicalSniper99, Andruwjones, and ShadowsofSatan
2 Thanks to XxB3asti3xX
3 Thanks to spletchie for the info
4 Thanks to Erathia96 for confirming