Hi Scott
A spell-casted loosing a one-segment spell such as magic missile is pretty hard to stop. All of my mage PCs have spells of one segment casting time for tight situations,
The physical attack must occur on the segment before a spell is cast to disrupt it--unless concentration is required to keep the spell going.
Multiple attacks don't matter, as the first happens on the segment of the round indicated, then another follows thereafter. So if the first of multiple attacks occurs before the spell is cast, and it hits, that's the one that matters most. Of course the second attack might be even more important, such as one that kills the spell-caster...
Cheers,
Gary