well cort said above, that he moveed the irradiated units out of the way, i do admit that the gaurds are pretty slow, but if he is THAT smart he will have them spread out with hotkeys to make sure they dont get irradiated and all die at the same time...your not ganna have enough enerygy with your vessels to irradiate that many gaurds and muta anyways...so its kinda pointless unless you have somewhere around 10 sci vessels with irradiate and upgraded full energy...
wraiths and scouts own pretty much all air...thats where i see zerg having a weakpoint, there air isnt the greatest, one, they dont have a REALLY strong single unit like the terran or toss...BC's and carriers....cardiens are pretty srtong but they cant attack air...scourges do help, but they cost more than they arre worth in my opionion...muta are pretty good if you have enough of em, but they ge tpwned at the hands of the might cloaked wraith...