You must perform 50 unique executions. The two things to consider are 1. enemy type and 2. the melee weapon you are using. Different melee weapons cause different animations on different enemies. You will want to mix up the melee weapon you are using and should not have any issues unlocking this. One extra thing to note is that bosses also seem to have their own unique animation so if you are not sure what weapon you missed one on going back to execute bosses might help you out. I got this without ever using the hammer so apparently there are enough of them it should not cause a big headache for most people.