Haohan's Vote V: Chief Yip-Yip

Don't be confused by Yoon saying he needs more time to prepare the mixture or by not having anything in your bags for this quest. Just head straight for the cave. When you arrive, the extra action button will appear, even though you don't have any poisoned melons in your bag.

Additionally, he takes double dmg when he's stunned by eating the melons, so time your CDs accordingly.

Edited to add: You need to pass 20k/21k revered with tillers before you can pick up this quest, which may required you to wait for another set of dailies between IV and V of this quest series. However, once you pick up the quest, you can head straight to the cave. Even though he seems to indicate that you need to wait at this point, you don't.

This is the final quest in the Haohan's Vote chain. You receive this quest once harvesting your Striped Melon for the quest Haohan's Vote IV: Melons For Felons.

You will find [npc=59567] in Kunzen Cave, along the northern border of Valley of the Four Winds.
The NPC has over 11 Million health, but you are aided by a "Throw Melon" ability on your Extra Action bar, which will light up when Chief Yip-Yip is about to charge. Simply click your action button to stun him.

@trailmix, if you don't want spoilers, maybe stay off of wowhead? This site frequently puts the entire quest chain in the "quick facts" box... If you come here, I can't see how you can complain about spoilers, as "spoilers" (info about how the quest works, where the mobs are, etc) are the whole purpose of wowhead!