Learn French: Cependant, de tels coups magistraux sont rares. - However, such masterful shots are rare.

In French, the plural indefinite article des usually changes to de when an adjective comes directly before the noun. Because the adjective tels comes before the plural noun coups, des becomes de.

How do we know to use tels rather than tel, telle, or telles?

Tel acts as an adjective here, meaning "such," so it must agree in gender and number with the noun it describes. Since coups is masculine plural, we use the masculine plural form tels.

Could I use tellement de instead of de tels?

No, they express different ideas. De tels coups means "such shots" or "shots of this kind" (focusing on the nature of the shots). If you said tellement de coups, it would mean "so many shots" (focusing on the quantity).

What is the singular form of magistraux?

The singular is magistral (masterful). In French, most masculine adjectives and nouns ending in -al form their plural by dropping the -al and adding -aux.