In Khralean vs. Khralean battles, I've had this tried against me, and tried it against others, and I've yet to see the strategy lose. Basically, build Swarmers almost every chance you get, possibly with an Infector in a safe spot to help heal. Once somebody has a 3-4 on the board, it's very difficult to counter.
There's only three units that can hit them sufficiently - Garudas, Wyrms, and Swarmers. Every time I've tried Garudas, 3-4 Swarmers will mass and combine to take out each Garuda. Meanwhile, when I attack, I get dinged a little bit every time, which adds up and makes it that much easier for the Swarmers to take it down. Wyrms are both vulnerable and difficult to protect.
So far, the best defense I've found is to counter with the same strategy. I've never gotten this far into a battle, but I suppose after ~10 full strength swarmers, better to add a couple Garudas and a Wyrm in there when you can finally protect them.
But this feels a bit cheeseball. Is there a better/more efficient counter?