Basically you need to farm a TON of salt until you have max Salt (9999) and progress the game to Chapter 7. Go to the Pixball token vendor behind the Hotel in Ester City and buy as many tokens as you can afford . Go to the Pixball machine at the dock and make sure that the top prize is the 'True Demon King'. If it isn’t the top prize reload the game and try the Pixball machine again until the True Demon king is the top prize. (You will win the Top prize about 1 in 9-10 uses of the Pixball machine. You’ll also get 3 tokens out of a Pixball sometimes) Each time you win the ‘True Demon King’ Pixball you get a Stun Grenade item in-game in Earth Born. It's a long grind but you will DEFINITELY beat Earthborn New Game(+) easily!

Once you finish purchasing Pixballs, progress the story to the point where you enter Solomon's grave and farm the salt bug near the entrance until you have 9999 Salt once more. Spend another 9999 Salt for tokens and once again farm True Demon King Pixballs when you return to Ester City.

The author spent ~20,000 Salt and ended up with 93 True Demon King/Stun Grenades and 15 Pixel King/Healing Scales...and used about 70 Stun Grenades to defeat the 4 Generals, the Demon King, the Demon King's guards, and the True Form Demon King.

PS- If you have trouble with the King's guards, save AP for a round or two (using Stun Grenades each turn) then hit them with the Teamwork Skill from the Princess.