Here is the full list of all 21 Elven Assassin achievements.
50 Head Shots
1K Fired Arrows
50 Orcs Killed
100 Fired Arrows
50 Window Shots
20K Fired Arrows
50K Fired Arrows
5K Fired Arrows
2.5K HeadShots
10K HeadShots
25K HeadShots
50K HeadShots
500 HeadShots
2.5K KilledOrcs
10K KilledOrcs
25K KilledOrcs
50K KilledOrcs
500 KilledOrcs
1K WindowShots
20K WindowShots
100 WindowShots