Here is the full list of all 26 How a Retired Strategist Saved the Country achievements.
Construct a building for the first time
Emerge victorious in battle
Clear stage 1
Clear stage 2
Clear stage 3
Clear stage 4
Clear stage 5
Clear stage 6
Clear story mode
Clear the game on Difficulty B without carrying over battle points or using the otherworldly castle
Clear the game on Difficulty C without carrying over battle points or using the otherworldly castle
Have at least 20 Sages during a battle
Have at least 100 Steel Gear Soldiers during a battle
Use magic research to strengthen your Magicians to +35
Have at least 3,000 Defense from Forts alone
Produce 10,000 Tea during a single battle
Raise your First Strike to at least 10,000
Have at least 100,000 Military during a battle
Reach year 3 without losing any soldiers
Clear a Free Battle with seed ”569216014”, skill points,set to 26, enemy strength 400%, Combat length: Long, and no otherworldly castle
Clear a Free Battle with seed ”569216014”, skill points,set to 26, enemy strength 400%, Combat length: Long, Forest Sage Lv4, and no otherworldly castle
Clear a Free Battle with seed ”569216014”, skill points,set to 26, enemy strength 400%, Combat length: Long, Iron Workshop Lv4, and no otherworldly castle
Clear a Free Battle with seed ”569216014”, skill points,set to 26, enemy strength 400%, Combat length: Long, Magic Laboratory Lv4, and no otherworldly castle
Clear a Free Battle with seed ”569216014”, skill points,set to 26, enemy strength 400%, Combat length: Long, Mobile Fort Lv4, and no otherworldly castle
Clear a Free Battle with seed ”569216014”, skill points,set to 26, enemy strength 400%, Combat length: Long, Territory Expansion Lv4, and no otherworldly castle
Obtain all other Achievements