Learn Turkish: Koro şefin yardımıyla pratik yapa yapa güçlendi. - With the conductor's help, the choir got stronger by practicing repeatedly.

The root is the noun güç (strength/power). Adding the denominal verb suffix -len (from the -lAn family) turns it into a verb that means "to acquire a state," so güçlenmek means "to become strong" or "to gain strength." Finally, -di is the past tense suffix, giving us "got stronger."

Could we use güçleşti instead of güçlendi in this sentence?

No, and this is a common mix-up! The root güç can mean "strength" (noun) but also "difficult" (adjective). When you use the -lAn suffix (güçlenmek), it means "to become strong." If you use the -lAş suffix (güçleşmek), it means "to become difficult." Since the choir is gaining strength, we must use güçlendi.

What is the grammar behind the repeated phrase yapa yapa?

This is a repeated action adverb (or gerund) created by adding -a or -e to a verb root and doubling it. From yapmak (to do/make), yapa yapa means doing it continuously or repeatedly. Combined with pratik, pratik yapa yapa means "by practicing over and over."

Why is there an -ı and a -yla at the end of yardımıyla?