Here is the full list of all 26 Heroine of the Sniper achievements.
Acquire all skills
Beat the game in difficulty level Easy or above
Get A rank or above in all missions of difficulty level Easy or above
Get S rank in all missions of difficulty level Easy or above
Beat the game in difficulty level Normal or above
Get A rank or above in all missions of difficulty level Normal or above
Get S rank in all missions of difficulty level Normal or above
Beat the game in difficulty level Hard
Get A rank or above in all missions of difficulty level Hard
Get S rank in all missions of difficulty level Hard
Get 8 critical shots in mission 1 (Difficulty level normal or above)
Beat Mission 2 without anyone noticing you (Difficulty level normal or above)
Beat Mission 3 with 100% accuracy (Difficulty level normal or above)
Beat Mission 4 without being noticed by anyone (Difficulty level normal or above)
Destroy all armored-vehicles in Mission 5 (Difficulty level normal or above)
Shoot down the rocket with rifle in Mission 6 (Difficulty level normal or above)
Beat Mission 7 without retries (Difficulty level normal or above)
Beat Mission 8 without firing more than 18 shots (Difficulty level normal or above)
Beat Mission 9 within 3 minutes (Difficulty level normal or above)
Arrive at warehouse without anyone noticing you in Mission 10 (Difficulty level normal or above)
Shoot the tire of armored vehicle in Mission 11 (Difficulty level normal or above)
In the last scene of Mission 12, finish the target off in one shot (Difficulty level normal or above)
Beat Mission 13 without taking damage, without retries (Difficulty level normal or above)
Eliminate all enemies on ground during Mission 14 (Difficulty level normal or above)
Beat Mission 15 without use of machine-gun, without retries (Difficulty level normal or above)
Get 200 critical shots