Disclaimer: I am not the creator of these characters, obviously, as One Piece and Naruto were created by different people. All copy-righted stuff property of their respective owners, etc.
Note: I'm using Naruto from the beginning of Shippuden, who is somewhat strong, and Luffy from before the current arc. That means no Haki, which could potentially stop Naruto from fighting before the match even began, after all.
Luffy vs Naruto
Luffy spun around, his leg sweeping across the entire field, bashing into Naruto.
"Oww!" Naruto growled, "You'll pay for that!" He pressed his hands together, creating an army of shadow clones. Dozens of Naruto's lunged at the pirate, punching and kicking, as Luffy went flying into the ground.
He laughed, amazed. "Wow, you can make copies! That's so cool!"
"Yeah. Too bad punches can't hurt me, or you'd have a shot at winning!" Luffy grinned, another sweeping kick destroying every single clone one after the other.
"You… you bastard!" Naruto glared, pulling out a kunai knife. Another thirty or so clones leapt out from the trees around them, throwing kunai and shurikens. Luffy dodged frantically, bending into impossible positions, only three blades hitting his stomach.
"Geez… those hurt! You'll pay for that, fox-boy! Gum-gum pistol!" He roared, a massive punch stretching across at Naruto, who fell back with a thud.
"Gum-gum gattling!" was all the ninja heard, before punches flew in every direction, hitting every clone and Naruto repeatedly.
Naruto grimaced, wiping the small trickle of blood that the censors allowed on his face, glad it made him look less hurt. He couldn't lose to this clown!
"Rasengan!" He cried, slamming a spinning ball of chakra right into the young pirate, who had no idea what was coming at him. Luffy corkscrewed around wildly, arms flailing as he spun on the impact of the attack. Faster and faster, until he smashed into the tree behind him, toppling it over. Naruto grinned, as the dust cleared. Then he noticed something: Luffy was trying to life the massive redwood he had landed on, arms straining with the effort! He lifted the gargantuan tree, swinging it around like a weapon. Naruto leaped in time, running across the trunk with another kunai ready, and swung at Luffy's head. Luffy leaned out of the way, his neck stretching several feet backwards, before rebounding back, right into Naruto's head!
"Oww…" Both kids complained, rubbing their sore heads distractedly, as the tree slammed down next to them, dust and leaves flying across.
"Lets get serious, strawhat." Growled Naruto, red chakra flowing across his body, the first tail of the legendary fox unfolding behind him.
Luffy grinned, "I'm always serious, Uzumaki." He swung his fist out again, Naruto dodging, faster than before. He pounced on Luffy, viciously scraping at him with the claws he had grown. Luffy cried out, as he drew blood from the shoulder, and bounced backwards. Naruto pursed him, as Luffy spread his hands over each other and made a gigantic net, smashing Naruto into the ground, with little effect. Naruto merely growled again, another tail unfolding behind him, as he clawed Luffy once more. He went in with a Rasengan, ready to seriously hurt his opponent, bouncing backwards in shock. What the…
Luffy had swollen up to a tremendous size, knocking him backwards before he had time to create the Rasengan.
"What is that?"
"Gum gum balloon!" Laughed Luffy, returning to normal. "And this", he gripped his knees, in a fighting stance Naruto had never seen before, "Is Gear second!" he cried, steam flying off of his now-red body. He sprang at Naruto, both attacking at once. With enhanced speed and fighting ability, both fighters seemed little more than blurs, knocking each other around repeatedly. They seemed equal in strength, as the various trees around them fell back from the force of their impacts, but neither fighter backed down.
"Take it back!!!" Screamed Naruto, clawing into Luffy.
"You first!" Yelled Luffy, kicking the young ninja hard.
Both fighters stood, alone in a now empty plain. The sunlight shone on them brightly, as they readied their trump cards.
"Lets end this once and for all… Summoning Jutsu! Where Naruto had stood, a gigantic frog with a sword posed, ready to aid him.
"Sounds good… Gear third!" Luffy swelled up larger than ever before, now the frogs size himself. There was a small stare down, wherein the editors put flashbacks, hoping to make this fight more dramatic while rounding the word count up to twenty thousand or so. Once that was done… they fought.
The frog grumbled to himself, rushing Luffy with a headbutt., as he swung. His fist collided, and they seemed equal in strength… then all of the air inside Luffy rushed to his fist, swelling it even more, sending Gamabunta back.
"Damn it, get him!" Yelled Naruto, unhelpfully, as his frog pulled his sword out. He slashed out at Luffy, who just bounced back in time.
"Wait, did you just talk to yourself?"
"Whaddaya mean? I'm up here!"
"On the frog? No fair! You can't get help!"
"Fine, then my crew can come help kick your ass!" Declared Luffy, folding his arms across in frustration.
"Yeah? Then my ninja buddies can help beat you to a pulp!"
The two boys began wrestling each other chilishly, yelling.
"We're not really going to get involved, are we?" sighed Usopp.
"That idiot! What's he think he's doing?" Growled Sakura, amazed at her friends stupidity.
"Listen, meats better!"
"Take it back! The only way to eat is ramen!"
"Nuh-uh! Meat even has the word eat in it!"
"Oh yeah? Well, Ramen has, umm, men in it! So there! It's for men!"
Their friends all sweat-dropped, watching the tussle. That was why they were fighting!?
"You know…" Both Zorro and Shikamaru spoke in unison, "They have beef ramen."
And with those words, a lifetime friendship was made, and they all lived happily ever after. (Well, at least until the TV ratings came in for next month, that is.)
A/N Thanks for reading! Reviews are love, let me know if you wanna see any other characters fight it out.