Hm, I am a bit scared by this positive feedback to my percentage-negation suggestion. I am not advocating against my own suggestion, I am just want to be cautious.
The way the archon works right now, in an all-or-nothing manner, was quite unique to the feel of the game. It emphasised the rock-paper-scissors battle mechanics of the game. The percentage reduction might remove a bit from the rock-paper-scissors flavor. Whether this is good or bad is yet another thing.
Then again, the way the unicorn currently obliterates cavalry on open is arguably a bit too severe indeed. Even a two-third reduction would still remain significant. Maybe a 50% reduction is already enough, which would leave the balance between unicorns and spiders/griffins unchanged.