Who should be assassinated / executed / otherwise incapacitated when consolidating power, to eliminate and prevent formation of opposition?
Seemingly obvious are leaders of the opposition, but after that things become less obvious: People who could potentially become leaders of the opposition. Any competent people who could potentially be useful to the potential opposition, not necessarily as a leader (but they could be potentially useful to your side as well). Who else? And how can one find these people? Answering these questions requires understanding how society operates and adapts in response to changes.
Inspired by real mass political executions, e.g., Katyn, in which the Soviets executed a great many competent Polish people, seemingly with considerable success: it took many generations for effective opposition to form.
Vaguely similar is brain drain, which also weakens a region by depriving it of competent people.