“He helped his servant Israel.” “The middle voice of antilambano means to take hold of something or somebody and in that way to help, and, like the verbs of touch, it is constructed in the genitive.” (Lenski). Here in Luke 1:54, “Israel” is used as a collective singular, speaking of all the people of Israel as if they were one person.
“remembering his mercy.” Figure of speech, metonymy (cp. Bullinger, Figures of Speech). “Mercy” is put for the act of mercy, being merciful. God “took hold of Israel his servant to help them,” in order to remember to be merciful to Abraham and his seed. In other words, God helped Israel in order to fulfill the promise He made to Abraham and his seed, a promise that they did not deserve, which is the point of saying that God remembered “mercy.”