Ashitaka's concentration was broken not by the first rock chucked at him, he vaulted 2 meters to the left to dodge that, but by his opponents insistence on a "youthful battle". What in the world is he talking about??
"You fool a shinobi must have a will of stone not be youthful"
Ashitaka was starting to have a harder time dodging the continuing barrage of rocks. He broke his hand sign and slammed his hands on the ground.
"Earth Release Barrier: Earth Prison Dome of Magnificent Nothingness"
A dome of earth, roughly ten meters in diameter, began to quickly rise from the ground around Ashitaka.
"Lets see your youth break through this!!"
Hopefully it would finish forming before his opponent got inside that way he could continue crafting his genjutsu. If not Ashitaka would then have to progress to his next layer of defense.