User blog comment:The ASL Pirates/Will Someone other than Luffy Fight the Strongest Opponent?/@comment-5690257-20150627142525/@comment-24.235.56.170-20150627173117

Zoro tried to fight Buggy, cut him and was stabbed as a result.

Usopp and the Usopp pirates briefly tried to fight Kuro.

Zoro fought, and was defeated by, Mihawk. (Inarguably, a more impressive antagonist than Don Krieg.)

Zoro and Sanji both attacked Arlong while Luffy was underwater.

And that's just East Blue. You can bet that at one point in any given arc, someone is going to try unsuccesfully to fight the main antagonist, either to show how strong the opponent is, or to show how brave the ally is. The point being made in this blog is that no matter who tries, or who helps out, it's always Luffy that beats them in the end.

Except for Morgan.

And eventually Mihawk when Zoro gets his rematch.

Further, Sanji had a similarly personal match (that Luffy stepped aside to let him fight) when he defeated Duval. (Just to illustrate my point about random other characters attacking the main antagonist, Hatchan tried to fight Duval before the Straw Hats arrived to beat him.)

Also, on the descent to Fishman Island, I'm pretty sure it was Franky who defeated Caribou. Though it may be said that Surume was the bigger threat at the time, in terms of crew vs. crew, this is another instance of Luffy not fighting the captain. And later, Caribou's final defeat wasn't even by a Straw Hat.

Of course, a few more exceptions to this rule are all the defeats that the Straw Hats have suffered:

In very much the same way that Zoro fought and lost to Mihawk, he was the one that faced Kuma on Thriller Bark - although he defeated the whole crew and Sanji did try to take Zoro's place.

On Sabaody, the only pirate to (arguably) win during the Straw Hats' encounter with the marines was Rayleigh when he interevened with Borsalino (aside from the entire crew defeating ONE Pacifista). All of the Straw Hats fought Kuma that day, and all of them lost (although Luffy was the last one to be defeated).

Luffy never actually defeated Magellan in Impel Down, and in fact the last person seen fighting him was Bentham.

At Marineford, Luffy was unable to defeat any of Aokiji, Sakazuki, Borsalino, Sengoku, or Blackbeard. And he was nearly killed by Sakazuki. Meanwhile, the major players in the arc were Whitebeard, who was defeated by Blackbeard; Blackbeard, who was chased off by Sengoku; and Sengoku himself.