Although I haven't read many novels which feature Leia dueling I have yet to find one that references her lightsaber style.
Is there reference to any media that states her style?
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There are multiple occasions where Leia uses a lightsaber in combat, notably against her brother (in training) in Ambush at Corellia and against Alema Rar on two separate occasions during The Dark Nest Trilogy.
There has never been any mention of which lightsaber style Leia uses, possibly because the idea of lightsaber "combat styles" is a relatively new invention from the Clone Wars era of the EU, possibly because they are a stupid idea in their own right, and possibly because Leia's training is haphazard and prolonged, keeping her from ever developing an established style.
I don't remember any specific mentions of her light saber form.
Also on wikia there is a Leia fighting Vader picture
This is a total guess: Form I: Shii-cho
Reasoning: This was the form taught to younglings and one would assume she might be taught it first. This image depicting a Shii-cho blaster deflect. http://starwars.wikia.com/wiki/File:Shii-ChoBlastDeflect.png