ROMEO Gentle Mercutio, put thy rapier up.
MERCUTIO Come, sir, your ‘passado’.
ROMEO Draw, Benvolio, beat down their weapons.
Gentlemen, for shame forbear this outrage!
Tybalt, Mercutio, the Prince expressly hath
Forbid this bandying in Verona streets.
[Romeo steps between them.]
Hold, Tybalt! Good Mercutio!
[Tybalt under Romeo’s arm thrusts Mercutio in.]
Away Tybalt [with his followers] (3.1.76-82)
And that’s it, game pretty much over. The fight can obviously be choreographed a bit, but it’s fast and furious, and perhaps frightened, especially at the end. Romeo tries so hard; he is clever in his appeals to both Mercutio and Tybalt as Gentlemen, addressing Mercutio in particular as gentle and reminding them of the specificity of the Prince’s decree. He appeals to their honour, for shame forbear this outrage, exactly the terms in which Tybalt has framed his own challenge. In this place, and this play, where names have been so crucial and so contested, Romeo addresses them both by name, repeatedly; he calls in Benvolio too, and they both intervene physically, as the (mostly editorial) stage directions suggest. It’s hard to think of anything else that Romeo could have done. But it’s not enough. And now it’s over, the end of the beginning, the beginning of the end.
The stage directions: Tybalt under Romeo’s arm thrusts Mercutio in is from Q1, which adds and flies, and a few lines later Mercutio will confirm that I was hurt under your arm. So the fatal thrust happens in the close confusion of bodies, Romeo as well as Tybalt and Mercutio, with Mercutio perhaps even physically restrained by Romeo, or Tybalt at least stabbing around him. Practicalities: although Mercutio has mentioned swords and Romeo explicitly named rapiers, what’s left implicit is daggers. A rapier isn’t, primarily, a stabbing weapon; rather, you fight with a rapier in one hand and a dagger in the other, for parrying and, eventually, stabbing. If Romeo’s blocking, then Tybalt might, conceivably, stab around him with the rapier, but if the three of them are grappling, then stabbing with the dagger would be more likely (and also probably safer on stage, easier to cheat a controlled, fake thrust).
Whatever, it’s over in 7 lines.