。
She competed against many great athletes.
Literal
She [topic-は] a-lot [genitive-の] wonderful athletes [with-と] competed.
競争する ('to compete') uses と for the rival or competitor — a comitative-flavored 'with' that frames competition as a mutual encounter. Pair this with 戦う ('fight'), 話し合う ('discuss together'), 結婚する ('marry'), all of which take と for the other party. 選手 ('player, athlete, competitor') is a general word for anyone competing in a sport or contest, and ~達 (with the suffix 達 'plural marker') makes it explicitly plural — though Japanese nouns are usually unmarked for plurality, 選手達 emphasizes 'multiple athletes' more clearly here than the plain 選手 would. すばらしい ('wonderful, splendid') gives the sentence an admiring tone.