Call of Juarez

The basic flow of the level is:
1) Ride down to the cemetery.
2) Bury the man and get the key from the boy.
3) Ride to the church and go out the back door.
4) Examine the rightmost grave to be shown the boulder that conceals the exit. (optional)
5) Examine said boulder to find it's too heavy to move. You then are shown another boulder perched on a hill. (optional)
6) Make your way to that second boulder -- climb the rock pile by the church back door, climb onto the roof, use the unhinged door to get up higher, and jump over to the ground that almost meets the roof. Make your away around the winding path until you see two outcroppings with a branch between them. Swing across. Then head right and follow the path, swinging across another couple of gaps, until you get to the second boulder. Go past it, up the hill and end up behind the boulder.
7) Use the whip to swing towards the boulder and jump. If done correctly you will collide with it and it will roll down the hill, nailing the first boulder.
8) Approach the newly revealed hole in the ground to end the level.

It turns out that you don't even need to do 4 and 5 at all. They are just there so you can figure out what to do without a guide. Skipping them will save some time. The time limit is not that tight, and you probably can grab the collectibles without worry. One is in the cemetery inside one of the picket fence structures, and the other is in a chest inside the church.

When I didn't know the basic flow of the level, I found the other solution fairly confusing. Reading it gave me the impression that you were to swing into the same boulder that covers the hole, for example. I hope this clarifies things a bit. I realize I still leave certain pathing details vague, but rather than get into laborious detail, here's a video I found on Youtube to make everything crystal clear. All credit goes to its creator tehgr8tec0rtez.

It's an easy achievement - you should be able to finish it in about 5-6 minutes. Jump on the horse, go to the cementery (sings will show you the way), talk to the boy in the right corner and help him to bury his father. You'll get a key. Go inside the church (you don't have to follow the road - just go through the mountains) and go through the back door. You'll see three graves. Stand next to the first one on the right and press X, then stand on the grave and press X once again (you'll see a boulder). Now turn around, climb the rocks on the left side of the church, walk through the roof and follow the long path to the boulder you've seen before. Use a rope and kick the boulder so it falls down and shows you a hole. Just get into this hole and that's all.