Europa Universalis IV
373 Achievements
200+h
GOG
My armies are invincible!
Gain at least 7.0 land morale.
1.9%
How to unlock the My armies are invincible! achievement in Europa Universalis IV - Definitive Guide
Gain 7.0 in land morale, most of this will come out of military tech padded with prestige, advisor bonus and military tradition. Should be more or less automatic if you just keep playing an European nation until the near the end and don't waste too much of your monarch point on none tech stuff.