就 here is a resultative complement meaning 'achieve / accomplish (as a finished state)' - same 就 as in 成就 'achievement'. So 练就 = 'practice to the point of mastery', stronger than plain 练习 (liànxí, 'to practice').
usage
Often takes a measure-word + skill object: 练就一身武艺 'mastered a whole set of martial-arts skills', 练就一手好厨艺 'trained up excellent cooking skills'.
