How can I ensure that I cut my nemeses' heads off?

Solution 1:

There is a way where, when executed properly, you can ensure you a 100% decapitation.

You have to unlock the Wraith Finisher Skill, that garantees you a decapitation (well, a head explosion...) after the entire combo.

To do that, tap E (on PC) to stun your victim, and then spam Right Click (normal attack).

Note that the finisher will only activate if your commander is low health, while always works on normal Uruks.

Sadly using the normal finisher, there's nothing that can garantee you a 100% decapitation.

Solution 2:

This is meant to be a percentage chance. The game was never meant to give you this option, since you could annihilate the whole army incredibly quickly. It's just a random event to help you stay on top of your game, without giving you exploitative powers.

TL;DR You can't, cause the game only wants it to be the only thing that can