How do you defeat El Gigante when Ashley is with Leon?

When you get to this area, before you fight El Gigante, use a Handgun bullet to bring the Boulder
down, then run past it and use the shotgun to open the gate, run into one of the huts and grab the KEY.
Run all the way to the end, use the KEY and EXIT the area. Return to the area, leave Ashley
behind, then go pick a fight with El Gigante. 2 Hand Grenades will make him bow down to you,
then you can climb onto his back, and if you are fast enough, Slice his Plaga 8 times.
Then after you land, throw 2 more Hand Grenades, climb on his back again, and slash 8 more
times and he will die.

The Rock does nothing except block the path.

Or better yet just buy the rocket launcher. One hit Kill

There are two ways to get past the second El Gigante fight.

The "wimpy" way - Shoot the boulder near the middle of the area, blocking his way for a short time. Quickly knife or shoot the chains on the door, and kick it down using the A button. Dash through, ignoring all of the huts, and do the same for the second door you see.

Once you get to the exit, El Gigante should have caught up with you, and you should make it out just in time.

The "warrior's" way - For this fight I recommend either the Red9 or Punisher handgun. Make sure either weapon's power are maxed as far as you can get them, and also make sure your reload speed and capacity is up to snuff.

What you do, as soon as you enter, run to the other end and shoot the boulder down. This will slow him down enough to buy you some shots.

Shoot him in the head until he removes the boulder, and then just play keep away with him, popping shots off when you can, and slashing his back when you can. (Note: Do NOT leave Ashley alone with the 'Wait' command. El Gigante WILL go after her.).

Also, sorry I forgot to add this!!

You can make his Plagas appear quite easily using Grenades. Incendiaries do damage, but not enough to make it appear any quicker. Also, Flash Grenades stun him, allowing you to get more shots in.

Also, you CAN use the shotgun if you want to...if you have any shells left from the house segment. The TMP is NOT recommended, and the rifle is too slow to reload.

If you still can't handle him, then purchase a rocket launcher and kill it with fire...and explosions.

Keep her with you and have a steady supply of ammo. It is not the path for the wary.

Easy! You don't even have to! Take out your ShotGun and shoot down the boulder, then get past all of the chain doors quickly (I use TMP for it. You can also use a HandGun), go in the 2nd house and get the old key, and then get out of there.