It's nothing but hard levelling in Elfland.

If that's what you want to do, go for it. Astos and the 4 Wizards are a bitch.
I don't even think there's any feasible way to hit Astos for any reasonable damage, and you'll just be praying for Critical hits. But I swear you may need to be between lv. 16 and 24.

Once you got the Key, the Dragon, Rune and Coral Sword will slide you into Ninja no sweat and the game'll be a cinch because your levels will already be kinda high.