The problem is you are mocking the idea that someone could hold that sort of belief.

Again, it's the 21st century, communism is an old ideology at this point. It's not feasible to argue this is a crazy position to hold. It's not just "Stalin," looking at self proclaimed communist and marxist or marxist-leninist states and you'll see a lot of tyranny. If you want to dive into philosophical or ideological anti-communist arguments this is evidence in their favor This sort of "communism/pseudo-communism in action" observation provides evidence in their favor (in that they claim to accurately describe the world) even if complete vindication would be impossible due to legitimate objections anarchists raise about Lenin's deviations/innovations.

There's a reason that modern communists (e.g. anarchists) see Lenin as a traitor to the revolution.

Ironically, this observation is why I think the original OP is misinterpreting the "mentor." It's a completely plausible shorthand to argue "Marxist-Leninism" is the root of what we're generally talking about especially for big picture world history surveys of communism in the 20th century. It's not an inarguable position but it's fairly normal.

I also think this really hurts your argument

Now, do I personally believe that Communism can ever be achieved?

So what about: "Communism can never be achieved and when people genuinely try to make communism in the real world they get sidetracked into creating a repressive authoritarian state" be wrong? IF it's not wrong can't simply rephrase that as "communism is TYRANNY." I think that should be seen as reasonable even if one thinks it's a bad choice. You can't force your opponent to narrowly hold the exact opposite position you hold. Dealing with ideological and philosophical arguments involves multiple different levels of arguments and multiple overlapping definitions for the same terms. Debates about communism that involve people across the political spectrum opens up a wider set of issues because what is accepted as a baseline assumption is dramatically shrunk.

To be clear: i'm not trying to deny the potential validity of a "not true communism" defense, it's just not the only reasonable starting point.