The good part of Diablo can be fought during the Evil Main Quest. In the Ice Temple, head East into the room with the scorpion. In the southeast corner, there is a false wall that will lead to some Ice which can be melted via Song. Use an Infernal Hammer or Infernal Pickaxe to smash the prison inside the room and be teleported to Good Diablo.
The strategy for this fight is similar to normal Diablo. He has all the same moves, however, he doesn't summon twice as many infernals after being weakened. Note that he has different items and stats as well