Brew an Awkward Potion
Add a Fermented Spider Eye to make a Potion of Harming
Add Gunpowder to make a Splash Potion of Harming
Add Dragon’s Breath to make a Lingering Potion of Harming
Add Redstone to extend duration only for other potions; it does not affect Harming
Add Glowstone Dust to make a stronger Harming potion version if available in your game version