Контртоп-спин сделать труднее.

In Russian, the verb to be is normally omitted in the present tense.

So instead of saying something like Контртоп-спин сделать есть труднее, Russian simply says:

Контртоп-спин сделать труднее.

That is completely normal. The idea of is is understood from the structure.

Because the sentence is talking about the action in a general way: to do / to make a counter-topspin.

A very common Russian pattern is:

[object] + infinitive + comparative/predicative word

So here:

Контртоп-спин сделать труднее
literally: A counter-topspin to do [is] harder

This is an impersonal way to say that performing that action is harder.

This is a question of aspect.

• сделать = perfective
• делать = imperfective

Here сделать presents the action as a single completed act: to pull off / to execute a counter-topspin.

That fits sports actions very well, especially when talking about one attempt or one successful execution.

If you said делать, it would sound more like:

• doing it in general
• practicing it
• the process or repeated action

So:

• Контртоп-спин сделать труднее = It is harder to execute a counter-topspin
• Контртоп-спин делать труднее = more like Doing counter-topspins is harder

Both are possible in some contexts, but сделать is natural if you mean one completed stroke or attempt.

It is the direct object of сделать, so it is in the accusative case.

However, because this noun is treated as masculine inanimate, its accusative form looks the same as its nominative form.

So:

• nominative: контртоп-спин
• accusative: контртоп-спин

That is why you do not see any ending change here.

Because труднее is a comparative form: harder / more difficult.

In this sentence it is not acting like a normal adjective modifying a noun. It is acting as the predicate of the sentence, so it does not agree in gender, number, or case.

Compare:

• трудный удар = a difficult shot
  Here трудный is an adjective and must agree with the noun.

• Сделать удар труднее = It is harder to make the shot
  Here труднее is comparative and stays the same.

Russian word order is much more flexible than English word order.

Both are possible:

• Контртоп-спин сделать труднее
• Сделать контртоп-спин труднее

Starting with контртоп-спин puts focus or topic emphasis on that specific shot. It is a bit like saying:

As for the counter-topspin, it is harder to do.

Starting with сделать is a bit more neutral:

To do a counter-topspin is harder.

So the difference is mostly about emphasis, not basic meaning.

Russian often leaves the comparison unstated if it is clear from context.

So труднее by itself can mean:

• harder than before
• harder than another shot
• harder than something already being discussed

If you want to make the comparison explicit, you can add чем:

Контртоп-спин сделать труднее, чем топ-спин.
= A counter-topspin is harder to do than a topspin.

So the sentence is not incomplete; the comparison is just understood from context.

It is a borrowed sports term, most likely from table tennis vocabulary.

It is written with hyphens because it is a compound borrowed term:

• контр- = counter-
• топ-спин = topspin

In Russian, technical and sports vocabulary often keeps this kind of compound structure.

Grammatically, it is usually treated as a masculine inanimate noun, which is why it works naturally as the object of сделать.

Yes. Сложнее is often a very natural alternative.

Compare:

• труднее = harder, more difficult
• сложнее = more complicated, more difficult

In many contexts they are close in meaning. In sports, both can work, though the nuance may differ slightly:

• труднее emphasizes difficulty
• сложнее can suggest greater complexity

So Контртоп-спин сделать сложнее could also be possible depending on the intended nuance.