Sona has quite a few mechanics that have to be used at the right moment. Every three abilities she casts and auto attack that has additional effects depending on what your last ability used was. For hymn of Valor, she deals double damage, Aura of Perseverance reduces the target's damage output by 20% for 2 seconds, Song of Celarity slows the target by 40% for 2 seconds. Hymn of Valor is used as a harrasment tool, Aura of Perseverance is for reducing mass amounts of damage incoming ( can be used to escape or in team fights), Song of Celerity is for chasing down low health victims( also can be used as a escape mechanic).
Cresendo is a stun that can be used to flee, save teammates, or initiate team fights.