Наставник считает инициативу сильной стороной, а компетенцию — зоной роста.

Because this sentence uses the pattern считать + X + Y = to consider X to be Y.

• инициативу and компетенцию are the things being evaluated, so they are in the accusative case.
• сильной стороной and зоной роста are what those things are considered to be, so they are in the instrumental case.

So the structure is:

• считать инициативу сильной стороной = to consider initiative a strong side
• считать компетенцию зоной роста = to consider competence an area for growth

Russian считать has more than one meaning.

It can mean:

• to count: считать до десяти = to count to ten
• to consider / to regard / to think: считать кого-то умным = to consider someone smart

In your sentence, it clearly means to consider / to regard, because it is followed by an object and a description of that object.

After считать in this kind of construction, Russian normally uses the instrumental for the role, category, or quality assigned to something.

So:

• считать инициативу сильной стороной = correct standard pattern
• сильная сторона would be nominative, which is not the usual form after считать in this structure

Compare:

• Инициатива — сильная сторона.
  Here the noun is in the nominative, because this is a different structure: X — Y.
• Наставник считает инициативу сильной стороной.
  Here Russian uses the instrumental after считать.

The dash shows that something has been omitted, usually because it would just repeat what was already said.

The full version would be:

Наставник считает инициативу сильной стороной, а компетенцию считает зоной роста.

Russian often leaves out the repeated verb in the second part:

Наставник считает инициативу сильной стороной, а компетенцию — зоной роста.

So the dash stands in for the missing считает.

А often links two things by setting them side by side in contrast or comparison.

Here the mentor is making two parallel judgments:

• initiative = a strong side
• competence = an area for growth

So а works well because it signals comparison/contrast.

• и would sound more like simple addition
• но would sound like a stronger contradiction

In this sentence, а is the most natural conjunction.

This phrase has two layers:

1. зоной is in the instrumental, because it depends on считать
2. роста is in the genitive, because it depends on зона

So:

• зона роста = area of growth
• зоной роста = as an area of growth

You can think of it as:

• зона чего? → роста

The head noun changes case, but the dependent noun keeps the case required by the phrase.

Because both words are feminine singular instrumental.

• сильная → сильной
• сторона → стороной

So the adjective agrees with the noun in gender, number, and case.

The same thing happens in:

• зона → зоной

This is a very common instrumental pattern for feminine nouns.

The word order is fairly flexible, but the version you have is natural and balanced.

Current version:

Наставник считает инициативу сильной стороной, а компетенцию — зоной роста.

This gives a clean parallel structure:

• object first
• then what it is considered to be

You could rearrange parts for emphasis, but it would sound less neutral. For example:

Наставник считает сильной стороной инициативу...

This is possible, but it puts more emphasis on сильной стороной.

So the given word order is a good default for formal, clear Russian.

Often yes, especially in HR, education, and professional-development language. But it can be a little broader depending on context.

Компетенция can mean:

• a skill area or competency
• sometimes an area of responsibility or authority

In this sentence, because it is paired with зона роста, it clearly means something like competency or skill area, not jurisdiction or authority.

Yes. It is very common in business, coaching, education, and feedback language.

It means something like:

• area for growth
• area for improvement
• development area

It is often used instead of something more direct like слабая сторона, because it sounds more constructive and positive.

So stylistically, the sentence sounds like professional feedback language.

Yes. Russian could express the same idea in a more explicit way, for example:

Наставник считает, что инициатива — сильная сторона, а компетенция — зона роста.

This version uses что and two mini-sentences with the dash pattern X — Y.

Your original sentence is more compact and elegant, and it is very natural in formal written Russian. Both are correct; they just use different structures.