How to deal with it with FS? Top of my mind - bait with a poison trap support. Demonvein is not tanky at all, and you will most likely be able to one turn. If you are on a 1x1 font battle, avoid confrontation until you guarantee you can kill it. Consider forfeiting the font for one turn or leaving a relic to avoid capping.
Another thing - if your BG have a steady amount of summons (boghoppers for FS mainly) you always can roadblock it and let it scale and shrine rush, provided the other player is focusing on buffing/keeping the superchamp alive...