I tried that a while back as well, but now I use:
/assist [mod:ctrl,mod:shift,nomod:alt,target=ToonA]
[mod:ctrl,mod:alt,nomod:shift,target=ToonB]
[mod:alt,mod:shift,nomod:ctrl,target=ToonC]
[mod:ctrl,mod:alt,mod:shift,target=ToonD]
/cast Lightning Bolt
/targetlasttarget

It worked much better for me this way. Was quicker response time as well.