The most efficient way is to reload your last save before the gná fight - this lets you try different difficulty settings without a full restart (because the game keeps your progress intact). Here's the interesting part though - while you can freely switch between most difficulty levels, the give me god of war setting is special, since it needs to be selected right when you start a fresh game. This is actually by design, making it a true test of commitment and skill from start to finish.

You'll notice each difficulty level changes how gná behaves and hits - she gets more aggressive and punishing as you climb up the settings (which is why some players like to practice strategies on lower difficulties first). Pretty much every difficulty except the highest one can be adjusted whenever you want through the settings menu, so feel free to experiment until you find that sweet spot between challenge and enjoyment. Just remember that if you ever lower it from give me god of war, there's no going back without starting fresh.