The Combined Attack is located in the bottom right of the
menu whilst in a fight. You have to move the analogue stick in the pattern and press
at the specified point. Each time you do this successfully it will count towards the 150 times needed. This is my strongest normal attack and therefore I use this pretty much anytime it is available. So for me, it wasn't something I had to aim for, it just unlocked as I played through.
Note, I am using a mace, so the Combined Attack may be done differently to how I describe it.