I also heard biting. Watch her lips, they definitely look like they're making a "b".
The thing is, the lip and mouth movements a person makes when speaking are a function of accent, in addition to the language they're speaking. For example, if a Canadian from Ontario says "later" their tongue touches the ridge behind his teeth twice, one for the "l" and one for the "t" (which may sound more like a "d"). When Karen Gillan pronounces the word "later" using the accent she uses on Doctor Who (which I presume is her native Scottish accent) it sounds like her tongue touches the ridge behind her teeth only once (for the "l"), but then she replaces the "t" with a glottal stop (essentially the throat closes) resulting in a pronunciation closer to "lay'er" (where the apostrophe represents the glottal stop---think of the pause represented by the hyphen in the word "uh-oh!").
In certain accents, the "b" and "f" sound very similar (in fact, in some languages the sounds are indistinguishable) and the mouth movements to pronounce the letters are likewise similar. I'm not too familiar with Scottish English, but if "b" and "f" sound similar with a Scottish accent, it would explain why it looks like Amy is saying "biting" when she really means "fighting", or vice versa. I think a native Scot, or a person familiar with speaking to Scottish people, could tell with more certainty.
If anyone is wondering, I would not be crushed if it turns out she really did say "biting". Actually, it didn't even occur to me that she could possibly be saying anything other than "fighting" until I looked at the discussion on this board. But considering so many people think she said "biting", maybe that's really what she said. As
Sci noted, only the script could tell us for sure.