To climb start with grabbing the walls with both hands than press and hold:

PS3/4: X

Xbox: A

Then start with punchgrab with one of your hands while still grabbing with the other than you punchgrab with the one you still were grabbing with and so on till you're out of stamina.

(THERE IS NOW AN OPTION TO MAKE CLIMBING EASIER, JUST GO INTO OPTIONS AND TURN OF STRENGTH GRIP IF IT ISN'T ALREADY OFF. NOW YOU SHOULD BE ABLE TO CLIMB AS LONG AS YOU WANT)