You just use the analogue stick to aim where you want to cut. Sure you can use square and triangle (if its PS3) of course to cut horizontally and vertically but when you need precision you need to use the sticks to aim exactly where to hit. Its how you defeat Sundowner as well. You need to aim precisely at the arms holding up his armour otherwise if you hit the armour itself it deflects the attack and blows you backwards.
How have you gotten to the end of the game not realising you can precisely control blade mode with the stick?