Mandarin with all ten rings takes on the following:
1. Defends Helm's Deep by himself
2. Takes on all the ring-bearers at the same time - all 9 Nazgul, the 7 dwarf lords, and all 3 elves who were given rings, as well as Talion w/ Celebrimbor
3. Sauron w/ the One Ring equipped
Books, movies, and games feats all apply. How does he fare in each scenario?