Might try setting up a diff macro in the same slot for all your guys with the mod command. So you just hit 1 and one casts. then shift, alt, ctrl, etc. Its an extra step but might work better. Or stagger fear in your normal cast sequence. I used a cast sequence for my mages and staggered slow in it. It went something like this.

/castsequence reset=target ArcaneBlast, Slow, ArcaneBarrage, ArcaneBlast, ArcaneBarrage, ArcaneBlast, ArcaneBarrage, ArcaneBlast, ArcaneBarrage,

/castsequence reset=target ArcaneBlast, ArcaneBarrage, ArcaneBlast, ArcaneBarrage, Slow, ArcaneBlast, ArcaneBarrage, ArcaneBlast, ArcaneBarrage,

/castsequence reset=target ArcaneBlast, ArcaneBarrage, ArcaneBlast, ArcaneBarrage, ArcaneBlast, ArcaneBarrage, ArcaneBlast, ArcaneBarrage, Slow

Thats not the exact macro but I wrote it all out so by the time it got to the end of the sequence all 3 mages had cast the same number of spells and the mob/boss always had the slow debuff on him.