When does spellcasting (fomulaiec and spon. not fast cast) happen in combat?
4th ed. - at the end of the round.
5th ed. - on the spell caster's initiative.
On the caster's initiative. That's just spd i'm afraid, so probably last.
That's why you always carry a loaden crossbow around (+7 initiative). 
Not if you master and Fast Cast the spell 
If you plan to take your magus in combat, always have a few Mastered attack spells, and cast your defenses before engagement.