'Strong heart', word for word. It reads like a primary school worksheet.

What a Japanese person actually thinks
Childlike. Not offensive, not wrong, just conspicuously simple — the register of a classroom poster about being nice to your classmates.

Meaning

Literally: strong + heart
Actually: Grammatical and comprehensible. It's also the exact phrasing a Japanese six-year-old would use — adjective plus noun, no idiom, no compression.

Scores

Naturalness●●●●●

Correct grammar. Adults writing about mental toughness would reach for a compound instead.

Coolness●●●●●

The hiragana い in the middle breaks up the visual block and makes it look like a sentence fragment.

Watch out

Nothing embarrassing lurking in here.

Get this instead

剛毅 gōki
fortitude; unbending firmness of character

The adult, compound version of exactly this idea. Two dense characters, no hiragana.
平常心 heijōshin
an unshaken mind under pressure

If 'strong heart' meant staying calm when it counts, this says it precisely.

On skin

強い心

Orientation: either

N/A — change the words.