NOW it starts really going down.

Nat now reveals that she was secretly the biggest munchkin of all.

Nat is only a "munchkin" in the sense that she specced for thieving and social situations instead of combat. She couldn't beat the boys in a fight but she can absolutely outroll them if she wants to use Persuade or Diplomacy to settle the fight.

Persuade, Diplomacy, or Sneak Attack.

Yes5

Nat uses diplomacy to make a psychological attack with sneak attck

You mean intimidate, right? :P

She wouldn't really need to be a munchkin, just have a correct specialisation. Nami is a Rogue, which means she has Sneak Attack. Now, a rogue without absurd specialisation wouldn't be able to fell dedicated front liners with a single sneak attack...

...but doing it after they've beaten themselves to near-death already is a different story entirely...

(Interestingly: to qualify for sneak attack, you must be flanking, which is defined as having an enemy threatened by a character who is friendly to you... hilariously, right now, it could be argued that Zoro and Luffy are both friendly to Nami AND are threatening each other, which means Nami has a flank on both of them.)

This is also 3.5, which means that Nat might be able to get away with some Nonlethal Damage shenanigans.

True true. Unarmed attacks are nonlethal. Unless you choose otherwise as a monk.

Or flat-footed.

Dat Rika epic face though. Pure 'why do I hear boss music'

"There is only a truth!" The mystery character behind all the killings around appear

By defeating both Zoro and Luffy together, did Nami get any exp and could level up?

The fact the the player where perfectly willing to go all the way despite knowing the situation makes this scene even funnier than in the original.