While in blade mode a red box will appear on the enemy, slashing through this will expose it's power core which you can grab by pressing circle. Raiden will crush it and this will count as a Zandatsu.
Failing to press circle will not count towards this trophy, you have to press circle afterward to see the animation of Raiden crushing the power core.
You cannot perform Zandatsus on dead enemies, it must be performed on live enemies.
Stronger enemies will need to be weakened or staggered before you can perform Zandatsu on them.
Aside from netting a trophy from performing Zandatsus, they're worth doing as often as needed in order to fully replenish health and FC.