Before facing Atlas, you'll come to a pair of high walls. Boulders continuously fall down from somewhere up above, making this a particularly treacherous climb. As such, you'll to do your best to dodge the boulders to avoid taking damage.
To avoid the falling rocks, stay near the gap in the middle of the wall and jump over to the opposite side each time a rocks falls. The falling rocks will alternate from the left and right sides of the wal, so as soon as one falls on the side next to you, jump over to that side because another boulder is due on your side. Push up on the Left Analog Stick and press the X button to quickly ascend.
When you reach the top of the first section of wall, switch over to the left side. Press and hold R1 to slide down the long stretch of wall. Navigate over to the second split wall. Same deal as the first wall, but the rocks fall a bit faster. This video is from our unofficial game guide. You can view the full guide here