Hi, just coming on here to confirm the issues everyone is having are also true for me. Tried all the tips and tricks without succeeding by myself. Eventually got tired of it and submitted it as a bug since: No clarity on how how to do the achievement, mine would fail without me knowing how many I did correctly and why. Also extreme frustration around targeting missing and line of sight issues.
In the end I managed to team up with a friend and got the achievement in 2 tries, still not knowing how we got it that time - obviously we hit the 10 targets correctly but we really still did not see and understand what we did differently from the try before. So my recommendation would be to go and do it with a friend. To the GuildWars 2 team I would say; please make it more clear when you hit a target successfully.