You must first break the Qutrub's right hand weapon. It will only use the skill while it no longer has a weapon. If you break its second weapon, it will no longer use Spinal Cleave, however occasionally it will put its weapon away and continue to fight using H2H.
Facing the target from the front seems to be the best way to break its first weapon. A random critical hit or multi-hit weapon skill will make it drop its first weapon. Dancer 'steps' seem to have a very high chance of breaking both the first and second weapon, this can be very helpful if you have a dnc friend to help you.