depends... if you don't care or don't want the trinket to affect your first cast
/castrandom xxxx
/use 13
/use 14
/cast Elemental Mastery

I think this also delays it for one cast so you have to spam the button more than once before the trinket fires ( not 100% sure though )
/use [combat] 13
/use [combat] 14
/cast [combat] Elemental Mastery
/castrandom xxxx