I'm thinking more of stealth, ambush, preemptive attack by a ranged martial 'sniper' class (rogue, ranger, ...), allowing them to subtly disable enemy spell casters from a distance/while hiding. Perhaps more a kind of unnoticed 'blowpipe dart/non physically damaging' kind of ammunition that goes unnoticed until they attempt to cast a spell when the attack begins...
e.g. 'arrows of miscast magic' , or something to the effect of 'arrows of attention deficit disorder'
- when hit, characters have an 80% (50% on successful save) chance of casting failure for spells that require concentration for 1turn/caster level.
