If you got EM, Haste doesn't make much sense because EM should be tied with Ascendance and 15% Haste suffices to bring Lvb to 1sec, Mastery in this build is the way to go.
Without EM, Mastery seems solid, you don't hardcast many spells as Elemental because you are often focused, most dmg comes from Fulmination and Instant Lvb's overall.
Altough few people go for Crit, Fulmination and EB crit pretty high, even if Lvb is unaffected by Crit, those 2 other spells do serious damage if they crit.