You have more time than you think with Blade Mode. When the boulder comes flying at you, gently turn the right stuck until it's at the same angle. Then, nudge the left stick to move the line up or down so that it lines up with the targets. All you need are a few of the targets to line up. Then you strike, by releasing the right stick. It will cleave the boulder, and a new set of targets will appear. Once again, turn the line with the right stick, then hold the right stick to keep the blade drawn. Move the line up or down with the left stick to align. Then release the right stick. And so on.
It's a little tricky, but certainly doable. If you feel frustrated, take a break and come back later. It will retain your checkpoint at the start of the final boss battle.
Another tip with the final boss -- did you upgrade to circle-dodge with X+Square? Make liberal use of this whenever the final boss flashes yellow in his attempts to clobber you up close. By circle-dodging and pressing left or right at the same time, you will leap to his side, sidestepping his attack and attacking him at the same time, and putting you in a position to get a few free hits in.
Then, when he lights up with fire, smack him once or twice and run away so you can dodge whichever attack he unleashes.
