Found from powerful enemies means only found from defeating epic, legend or mythic enemies. This also classifies it as a ultra rare legend. They are hard to find but nadroji can make finding them easier.
For shrine enemies, orange shrine is one way but to summon epic enemies from any shrine, you can use hunter ring or have the hubter perk from ascension. Also, you can get this nadroji ring on any difficulty since it doesn't say which difficulty the nadroji ring drops at.
Epic enemies means any epic enemies included. This means if you defeat epic enemies from shrine, epic enemies at end of floor, cartographer, enslaver and by feat, they all have a chance of dropping a rare legend such as nadroji ring.
Legend and mythic enemies also drop nadroji ring. Mythic enemy would either get an eternal nadroji ring or just legend drop. Why? It drops 1 eternal and 3 legends. That eternal has a chance of nadroji ring and the legends also have chance of nadroji ring.