How does Furious Swing works togheter with Precision?

It doesn't

It doesn't, you have to choose one, it's a grey area but hear me out.

RAW a giant with Furious Swing and Precision rolling one 6 means, for Precision, that he opponent doesn't roll defense dice and get 1 wound instead of rolling defense dice, while for Furious Swing he has generated hits (one) and deals X wound (X=D3+wounds on giant) instead of rolling defense dice.

Now we have two effect working instead of something, and being instead an alternative/substitute, both effects cannot apply to the same defense roll, cause the moment you apply one of the effects then you dont have a defense roll anymore and you can't apply the other one.