I had a mutton shoulder and I wanted to prepare it in a way which imitated to some point doner kebab, so the meat was sliced into thin pieces and fried on a pan. However it was a bit tough.  I'd marinaded the meat for 24h in sour milk and spices - it didn't help much.  The meat wasn't low-quality and it tasted very well, the only issue was it wasn't tender. Any ideas what to do about that? 
Doner kebab is A) not a single joint of meat and B) cooked in slices, so attempting to get anything similar with the technique you described is a triumph of optimism over reality I'm afraid.  As Gary suggests, lamb/mutton shoulder requires long, slow cooking, but this will not yield a doner-like result in terms of texture. I would rub the lamb with plenty of spices, cumin, coriander, garlic etc and olive oil. Make small cuts in the meat and really massage the spices in. Place the lamb in a roasting tin and cover with foil.  Get the oven as hot as it will go, put the lamb in, and turn it down to 150C. Cook for 4 hours and the meat will fall off the bone. Pull to shreds and serve in pitta breads with yoghurt, salad, onions and chilli sauce. Voila! 