I've just run a fight with a big bad in their lair being assaulted by the PCs. My big bad (a Hecatean Hag) not only survived the first round, but got away with 'nary a consequence. The key, I found, was in the minions. I had a handful of cultists whose only purpose was keeping them away from their mistress while she finished her ceremony (half of them providing Blocks, half attacking) and a bunch of spirit hounds to keep the PCs attention. Also, as has been noted, in Aspects on the zones and scene. The Aspects HARVEST MOON and AUTUMN EQUINOX made for some powerful magic getting thrown around. In the end, I had two PCs taken out and the remaining two happily accepting a concession from the Hag after her ritual failed, allowing her to escape.
As a side note, if you are worried about over-use of Aspects per exchange, you could institute Diaspora's context system, where you can only invoke one aspect per context per roll. So, one personal Aspect, one scene Aspect, one zone Aspect, one Aspect on the target, etc.