charcoal grill is an art so avoid it and go with the gas grill, preheat the grill to 400 and then lower the heat, place the tikka, close the dome and resist the temptation to open the lid or flip the tikka for 6-7 minutes. Flip and grill for another 4-5 mins and then turn the grill off but let the tikka sit for another 2-3 mins.
Oh and always use leg and thigh piece combined. Drumsticks are good for frying maybe but not for grilling.