This does suck, i tried multiple times in the end I finished the whole story then went back, for this achieve you get two very very short windows to pull off the /kneel, its that short a window you have to already have the text in your chat window ready to just hit enter I even copied it to clipboard so I could quickly paste it back in chat if I missed the first opportunity , TiffyMissWiffy has a video on YT called "Guild Wars 2 YOUR MAJESTY Achievement" That shows how to do it very easily