When you've saved Rosalind from the zombies yet again and either killed or ignored all of the zombie cows, you can proceed to the next checkpoint in the O'Bannon Farm level. Head up the hill towards the fence to proceed. Along the way, be on the lookout for a special named lollipop to your left. It will be hiding behind a rock. You can find it from its telltale glow and the jingling sound it emits. Proceed up the hill and you will find Rosalind in her bus. The zombies will be at her heels again, shoving rocks down the surrounding hillsides in an attempt at killing her off for good.
In this section, you will need to protect Rosalind's bus from the falling rocks. You will have to do so by blasting the rocks with your Chainsaw Blaster before they make the short trip down the hill and into the side of Rosalind's bus. The first rock will come from the right hand hill, the second rock will come from the left hand hill, and the third rock will come again from your right. Quickly alternate between both sides of the bus with your Chainsaw Blaster to ensure that Rosalind stays safe; if the purple health meter on the lower portion of the screen depletes during this section, you will receive a Game Over and Rosalind will die. If you are having trouble taking out all of the rocks in time, try feathering the trigger and letting the auto lock on take care of the aiming for you. When you have successfully protected Rosalind's bus from three rocks, it will jet off to safety behind a tree in the distance. The tree will fall as Rosalind passes it, barring your progress down the path ahead.
Approach the tree and saw through it using your chainsaw. Hit the chainsaw button and then swipe the control stick down to chop straight through the tree. Juliet will reach Rosalind on the other side of the path. Grab the Chainsaw Blaster ammo from behind the bus, and then approach the back of the bus and hit either the B or the O button to hop on top of the bus. You will now initiate another protection sequence. It seems that Rosalind is still inside of the bus, but something is wrong. Even with Juliet on top, Rosalind begins to drive through a boulder strewn canyon. This area is highly dangerous for the both of you, not to mention the defenseless Nick hanging from your waist. You will again have to protect the bus from falling boulders, and if you screwed up the last section too badly, it can very well come back to bit you here. The bus' health bar will be the same as it was when you last saw it, so if the bus took too much damage last time around, it'll be as good as dead in this more challenging sequence.
Don't hesitate to pull out your Chainsaw Blaster. The first rock that you will have to shoot down will come from directly ahead of you, and above. A zombie will push it off of the rock walkway that arcs over the bus. Shoot the boulder, and then quickly turn to your left to encounter the second boulder. It will be falling from the cliff side above you. Take it out as well, and the bus will start to move. Look above you on the rock walkway to find two more boulders falling towards the bus. Take them both out and the bus will move beneath the walkway. As you go ahead, look towards the ground to find two zombies attacking the bus from the ground. Take them both out now, and then immediately look up. You will be passing beneath another rock walkway, from which two zombies will be pushing two more boulders down on to you and the bus. Shoot these rocks, and then turn your attention back to the ground, where two more zombies will be attacking the base of the bus. Shoot them as well. The bus will round a corner ahead and, with a speedy boost, deposit you back on the ground, just a little disoriented.