WHAT? I POSTED HERE... ah well, ill have to rewrite it
allright. First, make the good old barrage spell.
register when the hero attacks.
set an integer called int ( i always find such a variable extremely useful ) to a random number between 1 and 100.
check if int is <= ( YOUR PERCENT CHANCE ) and if it is, add barrage to the hero, wait .2 seconds, remove barrage.
if that is not a long enought wait ( it will really be ~.35 seconds ) then experiment with increasing it / decreasing it ( if it is too long ) until you get a wait time that works.