The Nightmare Boss Fight Tips
All you really need to beat Nightmare is around 2000 Mag, 1700 HP, 3000 MP. And roughly 300 to 500 in the other stats. Afterwards, choose your summons carefully. You can fit around five or six onto the field.
Then time when you block, and use magic shield. Do this along with Shadow Shield.
Note, there is a chance to lose if you do not time your magic shield and block. As it can take you out in one hit.
Also, ensure to have Ralph’s Quests completed to get some extra stats. As well as Reina’s. This will make it easier.
The weapon I also used was Flaming Starlight White Wood Staff.
Also, to raise stats a bit faster, hunt dragons and other large bosses. They drop materials that gives a good boost.