Using trees as obstacles does slow them down. Watch out for falling trees, but that gives you another hidden advantage because you can jump over logs while they have to walk around. If the forest is dense enough you might even lose aggro and have to go back and get their attention again. Don't be in a big rush. Dispatch the adds as you run around and then line up your shots. The troll may turn erratically as it tries to avoid obstacles.

But I generally only do it that way if there's no handy ditch to shoot across.