Posted: 13/06/2014 at 20:06
Personally, I would dig the whole lot up and let it die before composting it. Make sure you get all the roots out. if you must have it, put it somewhere where it won't take all the moisture/nutrients from around your favourite plants. Get it away from your Acer asap. This appeared in one of my large pots without me noticing probably a seed dropped by birds, and took over the whole pot/plant, it's a real thug. I think they look good around the margins of ponds.
Looking at your almost dead shrub, when was it planted ??
If it is fairly recent, you need to keep watering it regularly for at least 12 x months or so until it is well established. Perhaps where you've put the Sambucus is too dry an area for it.
Hope you manage to sort something out soon, good luck.