Not Sure if anybody is still looking for this trophy
Just completed the game 100% last night and I was not able to get any useful info online and figured out the achievement myself (or may be I was just a bad searcher
). In any case, some tips for the Master Marksman Achievement are -
1. Use "Cat eyes" mutation from BaW expansion - boosts the CrossBow damage a lot (9-10 times I guess), along with a 50% Crit chance boost for CrossBow
2. Unlock and equip the "Lightning reflexes" (Combat tree) for the time-slowdown during CrossBow aiming - helps a lot (almost freezes the enemies)
3. Unlock and equip "Steady shot" (General Tree) for the additional 25% damage
4. Make sure you have as much "Attack power" boost as possible - Feline gear, Greater Chernobog stones (make sure you sheath the sword for this effect), Thunderbolt, Greater Red mutagens with Combat skills etc - any % increase in attack power, increases CrossBow damage. Finally craft some "Precision bolts" from any Blacksmith (if you have the diagram). Otherwise also fine, you can do with the basic bolts (might require one additional bolt)
5. Since this achievement does not depend on the difficulty level, try doing it in the "Just the story" difficulty to one or two shot opponents.
6. Use Devil's Pitt for the achievement - it would be better if the enemies are sufficiently lower level than you. Optionally you can use Axii but with Lightning reflexes maxed out, this is not that important.
7. With all the above, I just completed the achievement in under an hour (just search and kill everyone in Devil's Pitt - around 9-10 bandits per visit). Travel to Vizima and back to Devil's Pitt for respawn and then repeat the process
Hope this helps for someone who is still looking forward for the achievement
Happy hunting