Here is the full list of all 20 Peninsular War Battles achievements.
Destroy 3 enemy units in a row without losing any of your own.
Destroy 8 enemy units in a row without losing any of your own.
Destroy 12 enemy units in a row without losing any of your own.
Complete 5 scenarios on Hard difficulty.
Complete 10 scenarios on Hard difficulty.
Complete 15 scenarios on Hard difficulty.
Complete the tutorial.
Complete the Invasion Campaign as British
Complete the Invasion Campaign as French
Complete the Assault Campaign as British
Complete the Assault Campaign as French
Complete the Conquest Campaign as British
Complete the Conquest Campaign as French
Complete the Engagement Campaign as British
Complete the Engagement Campaign as French
Complete the Hammer Campaign as British
Complete the Hammer Campaign as French
Complete the Bonus Campaign as British
Complete the Bonus Campaign as French
Complete 30 scenarios on Hard difficulty.