Oh, tricky. They are just really good at abusing quick movement to avoid attacks, aren't they? With Marines dodging Lurker spines, avoiding the hits of Ultralisks, using Vultures to hit and run... the list goes on.
hm. i always countered medics with siege tanks, reavers, and dark templar...never thought of using spells cause medics have that pesky restoration and healing. in my personaly opinion broodling is a waste to use on a medic, why not save it for SCVs or siege tanks?