Merge & Blade

To earn this achievement you must have upgraded the academy to level 9 and match 3 or more of the lower level troops until you create the general.

These are the Academy upgrade costs and unlocks:

Academy level 2 – 0 gold unlocks rogue
Academy level 3 – 500 gold unlocks archer
Academy level 4 – 1,200 gold unlocks priest
Academy level 5 - 3,100 gold unlocks spearman
Academy level 6 - 7,800 gold unlocks barbarian
Academy level 7 - 19,000 gold unlocks alchemist
Academy level 8 - 48,000 gold unlocks wizard
Academy level 9 - 122,000 gold unlocks general
Total gold = 201,600

This is the order of troop upgrades:

Farmer – Blacksmith –Footman – Rogue – Archer - Priest - Spearman - Barbarian - Alchemist - Wizard -General

*I was able to go back to the first defense level with the difficulty of easy and earn this there once I had unlocked the relevant things.