Sneak in Giant Valley using Sylvan Shadows heading south past all the drakes and trolls and such. None see through sneak/invis, but be careful. Slow and steady will win this race.
Look for Onodrim (NPC Wood Troll - non aggro), and the Heartwood is laying on the ground right next to him. Pick it up, quickly port yourself out and you're good to go.
If you are using the MEHQ map, it is on the map next to Orodrim that says
Black Huorn Heartwood (Correct)
and NOT the southern-most one that says
Black Huoron Heart Wood The Heart of the Wood Part IV
Instead of fighting, use an ability to get past the trolls. I did it solo. I just used the racial ability "Sylvan Shadows" and just walked *behind* each of the mobs. Took all of 6 minutes to get the Heart of Heuron Wood (or whatever it is called). Then just used my Hunter travel ability to Esteldin, crafted the bow, traveled to Bree, hopped on a horse to Trestlebridge. Total time, less than 45 minutes without drawing my bow once...good luck! :)