Shivering Touch spell. 3rd level. Necromancy touch spell (Frostburn). 3d6 Dexterity damage for 1 round/level. No save. No lower limit. Thus, you can effectively paralyze the FB with one casting & a lucky roll or nigh dead-sure with two castings.
Normally, if there are two wizards in a party, you'll cast this in combination with Spectral Hand, get in your touch attacks - bang, the encounter is over before it even started.
That's the easiest way.
Others might include your Druid sneaking up on the FB in Wild Shape (wooded terrain?) and cast something nasty at the FB - she willl fly in a frenzy, never knowing that the squirrel up in the tree she can't even see hit her with some nasty Flamestrike.
Once the frenzy is over, the invisible members of the party close in and kill her off.
Since the druid-squirrel will remain at the site while she plays out her frenzy with trees and bushes, this is no new encounter and she can't frenzy again once the party comes in and kills her off.
Multiple Grease spells might be nice as well, since she can't use Balance checks while in frenzy, and she'll have to pass a save every time she swings once her axe is greased up as well.
There are many more options of course.
FBs are ridiculously easy targets for a party with multiple primary spellcasters & warriors.
They are only dangerous when met totally unprepared.