That actually helped. I've spammed the skill targeting the spot at the front while under the Kralk, the glowing horn weak spot on the left wing and actually hit nothing on the right one. But got the achievement.
Have you tried running it from the terminal with -forwardrenderer ? Maybe I'm just imagining things but it appears in my case at least to be more stable and run smoother./Applications/Guild\ Wars\ 2\ 64-bit.app/Contents/MacOS/GuildWars2 -forwardrenderer Thanks for the hint. Have looked at command-line options page. Have set dx9single and forward rendering. Looks better.