How to assassinate El Diablo on Mission 24

On mission 24, El Diablo can be killed instead of letting him run away, and subsequently the tunnel won't cave in, you can go through it to avoid the lava zone.

Get all the characters moved to the point

As the picture shows, move them to the sector as near the enemies as you can so that all of their guns can aim at the targets, and make sure not to scare El Diablo off (You may use F5 & F8 a lot).
After that is the key step of this assassination -- to make full use of QUICK ACTION:

• Kate's gun on the left enemy;
  
• Sam's gun on the upper enemy;
  
• Doc's ordinary gun on the lower enemy;
  
• John's gun first on the left enemy, then the other two;
  
• Mia's firecracker to the point in front of the two enemies. (This step may be debatable because the firecracker usually doesn't seem to be effective.)
  
  When you've done it, get Mia's firecracker and all others' guns ready (Raise them one by one), then switch the character to Doc and aim the sniper at El Diablo. You should definitely save now!
  
  The following steps are crucial ones which rely on your reaction speed and a little bit of good luck:
  
  1. Put your thumb on the SPACE BAR and middle finger on the "2" key (left hand of course);
     
  2. Snipe El Diablo with Doc then fast press the space bar several times and switch the character to Sam quickly by pressing "2";
     
  3. Right now Sam will still be raising his Winchester, and at least one of the enemies has survived, so send him to hell with your gun immediately!
  
  You probably need several tries to achieve it, just be patient.
  

Through the tunnel

Yet the death of El Diablo won't change the story after all, he will still show in Mission 25 (his twin brother for sure) , however it will affect the tunnel's cave-in.
If the characters go through the tunnel without El Diablo's body, nothing would've happened, otherwise carrying the body can still trigger the cave-in event.

All is done!

So this is it, maybe you have more efficient ways to do this or more secrets to share, I'd be glad if you leave the comment.
And I hope this guide is helpful to you, thanks for reading!

[Nullpunkt]

thanks man will try it out

[LE_MALEM]

I did this on live stream albeit a little differently than your method!