Ok, I got into another competitive situation tonight and learned a little more about this.
First, I do not have to hit the mob twice with a spell, just once. Further, from what I can see it works 100% of the time if I manage to start casting the spell before the opponent starts attacking with their weapon. Then, if they hit the mob first, they get damage on it, but after the spell actually hits and does damage I end up getting the kill no matter who lands the most damage or the killing blow.
It seems, from that and other experiences, that a spell takes effect as soon as it is started. The casting delay seems to have more to do with a concentration component to maintain a spell, or maybe just to prevent rapid chain-casting.
I am sharing this info because I don\'t even remember the last time I competed with another caster. But if someone with level 30 in any Way was to compete with me then we might learn more. I don\'t know if this is specific to Summon Missile of the Crystal Way or if it is a general thing with magic.
Also, I am less sure that magic itself is bugged and more certain that this is a design feature. The bug may be in the opponent getting messages that they are hitting the mob when they shouldn\'t be able to. Or, conversely, magic is bugged, and the spell should not take effect until the casting delay is finished.
Sorry, that was a bit long-winded.
Kaithro