Yes it's damn hard to counter. You need to surround the zealot with SCVs, press 's' (stop) to screw with the AI, the zealot will stop attacking the SCVs and try to hit the marine, and when he manually targets, pull back the SCV that is being hit (a zealot needs 3 hits to kill a worker). But it's damn hard. As you can see even this top terran couldn't do it, and they weren't playing for scrubs either. This was the finals and first place in GSL is $85k, second place $27k.