Elena with her max ranked cavalry summon is a start, as you can summon them directly on the backline where the spiders are, hopefully giving you a little time to clear the frontline. If you place Cavalry on the sides, hoping they bypass the scorpions in the middle and hit the spiders on the far right and left, then summon Elena's cavalry in the middle, it causes them to at least have to focus on the cavalry before decimating your entire frontline. Spec her with improved cavalry and they'll do heavy damage. Pair that with a crusader build Virion who can rezz your swords and boost their damage, as well as an Archer spec'd Avalon who will keep your archers doing insane damage and that's about the best counter I've found.
The problem I end up having is my humans still have to traverse the rest of the field to get to them, and they are spitting at you the entire time if your cavalry didn't manage to kill any. And I didn't even go into if they field Cleo/Rose together. Cleo's black hole can wreck the strategy by stunlocking your heals and potentially pulling the cavalry off the spiders. Then her eclipse will trigger for 8-10 seconds, making your damage very low, all the while Rose and the spiders will be spawning spiderlings confusing your human AI.