This strategy worked for me as well, twice. I did it with my 70 Human Rogue, and then with my 70 Night Elf Warrior. The first try with the Rogue was perfect, I attacked the fel reaver in front of the group of 5 elites on the east side of Thrallmar, the fel reaver killed me and the elites finished him off. I rez'd, came back just as the elites were finishing him off, and when he dropped I ran up and fired the mold. Perfect.
It took two tries with the Warrior, the first time I died too close to the Elite guys and when I rez'd they attacked me again. The fel reaver disappeared before I could fire the mold. Second time worked fine though, but there was about a 25 minute respawn time for the fel reaver to come by again. Cool, thanks for the tip.