Well, I suppose this doesn't matter anymore since Izual is demon soup already, but for future reference, I'm pretty sure Crushing Blow only works against regular monsters and minions. The CB function checks specifically if the target/monster is flagged as "special" (or however to call it).
Otherwise, CB is fairly strong, as it halves the monster's life. You won't leech from CB though -- it directly modifies the monster's life, so it is not applied as one of the damage types. (Not that this
really matters for you, as CB only works with melee as I think you already figured. They really neglected ranged attacks.
)