Story Archer-
I'm not familiar with Runelords- I'm more of a player than a GM and we're still making our way through Age of Worms.
But the way to make things more difficult is to turn the character's strength into a weakness. If at any time the BBEGs figure out what her capabilities are all they need is a "dead spy". They give misinformation to their mooks and send them off against the party. The party enchants them and gets incorrect info. The BBEGs can use this to do all kinds of nasty things- ambushes, sending them off to do their dirty work for them, etc. Essentially they can start to make the characters react to them instead of reacting to the
Information is only as good as it's source. If the party starts being slightly gun shy as to the source of the info it won't be used quite as much. And then when they figure out a way around that it's even more satisfying for them!
Sounds like a great group of players- good luck!