This achievement can only be unlocked in the sixth story chapter, Amon Hen.
The beginning of the chapter focuses on Boromir trying to steal the One Ring from Frodo. While playing as Frodo, take the level slow and do not step in front of Boromir at any time. If necessary, play the level a couple times to get the feel for it. Once you know how to complete this portion level, doing it without being caught by Boromir won't be difficult at all, as the only way you can be caught by him is by entering the brush he stands in. The achievement will unlock after this segment with Frodo and Boromir.