in a macro this is difficult to do, because if you put the /stopcasting befor the /castsequence all Shamans would stop casting and not only one at a time.
To solve this i would go from macro-round-robin with '/castsequence ,,Thunder Storm,' to software-round-robin (in keyclone/hkn,ahk...) and use a simple:
/stopcasting
/cast Thunder Storm

on all toons...