I’m not a tankie, but I don’t think that the idea of justifying political repression in Russia is likely to be much of a challenge. The Bolsheviks justified single-party rule and their own political repressions for a long time. If you’re a tankie, you’re presumably already willing to accept that.

https://en.wikipedia.org/wiki/Vanguardism

Vanguardism in the context of Leninist revolutionary struggle, relates to a strategy whereby the most class-conscious and politically “advanced” sections of the proletariat or working class, described as the revolutionary vanguard, form organizations to advance the objectives of communism.

The notion of a ‘vanguard’, as used by Lenin before 1917, did not necessarily imply single-party rule. Lenin considered the Social-Democrats (Bolsheviks) the leading elements of a multi-class (and multi-party) democratic struggle against Tsarism.[7] For a period after the October Revolution, the Bolsheviks (now renamed the Communist Party) operated in the soviets, trade unions, and other working-class mass organisations with other revolutionary parties, such as Mensheviks, Social-Revolutionaries and anarcho-communists, and local soviets often elected non-Bolshevik majorities.[8] Lenin did consider the Bolsheviks the vanguard insofar as they were the most consistent defenders of Soviet power (which he considered the dictatorship of the proletariat or ‘Commune-state’).[9] However, the situation changed drastically during the Russian Civil War and economic collapse, which decimated the working class and its independent institutions, and saw the development of irreconcilable conflicts between the Bolsheviks and their rivals. At the 10th Congress of the Russian Communist Party (Bolsheviks) in 1921, the Party made the de facto reality de jure by outlawing opposition parties and formalising single-Party rule.[10]

The impetus for having a vanguard party was used by the Bolsheviks to justify their suppression of other parties. Their rationale was that since they were the vanguard of the proletariat, their right to rule could not be legitimately questioned.