Here's a sample intro for Why Stalin was able to "surpass" Trotsky (similar to your question):
Stalin’s opportunism, ruthlessness and cunningness resulted in his triumph as the undisputable leader of the USSR by 1929. Though general assumption was that Trotsky would be Lenin’s successor, by exploiting his political position and social changes after years of war and revolution, Stalin was able to successfully promote his ideological vision of ‘socialism-in-one-country’ over that of Trotsky’s ‘permanent revolution’, and outsmart his political rivals to emerge as leader of the Bolshevik Party. This is most evident through the changes in society, differing ideologies of Stalin and his opposition, the role of personality and political tactics, which secured Stalin’s control of the Communist Party.
Hope the intro helps in forming your thesis, my general feedback is to make it more argumentative and keep Trotsky as the centre of your argument. Also maybe discuss his failure to develop a 'cult of personality' as he was not as an efficient and charismatic leader as Stalin was --> also how Trotsky may be vulnerable etc (his writings/books/speeches were criticised by political opposition as diverging from his party's values etc). I think it is a good idea to include concepts such as Marxism and political allies usually see each other as a threat - so I don't recommend saying that (maybe link it to the actions of Stalin...something like, Viewing Trotsky as an inherent threat, Stalin actively suppressed Lenin's Testament that made it increasingly difficult for Trotsky to consolidate power...)