EPTFor example, mutas and lings work great together because goliaths will start targeting the mutalisks, which are very tough and take half damage from goliaths but deal little damage, and ignore the zerglings, which are very fragile but deal alot of damage. Therefore: Muta//ling >>>> gols. But what if the terran player uses two science vessels and vultures too? the vultures will always target the fragile lings, the irradiate spells will make the mutas goliath dinner.
so it's all about what unit mix owns what unit mix, so you should just try your best to be the one with the better unit mix. I know, it's not the easy counter you were looking for. (if there was such an easy counter against everything the game wouldn't be very interesting at all)
instead of asking yourself what's good against mech, ask yourself "Why don't terrans like to mech against zerg?" and come up with this list:
1. At the start of the game, any mech build, even the cheesy rax before supply -> bunker rush, should be less reliable and less powerful than the 2rax m&m rush builds. What does that mean? It means that you can get away with building less sunken colonies and zerglings while still fast expanding. And every sunken colony and zergling you don't make, is an extra drone and it gives you quicker lair tech..
2. The terran player will use valkyries or science vessels against mutalisks, both of these take a long time to tech to. Therefore, you should always start out with mutalisks. Only turrets can save him and those can't attack your base and cost him money..
3. Terran players can't change their unit mix as quick as a zerg player can, because every hatchery keeps working when a zerg player changes his unit mix while barracks or factories or starports become idle when terran players change. therefore you have the advantage in this very important aspect of mech vs zerg. The art is for you to always have the right units mix against his unit mix..
4. When you have plague, the terran player is basicly history. Therefore you don't even have to feel pressured to finish the game quickly
