The right way to get her voted out isn't via a Republican challenger (at senate level, California is a single party state), it's through a combination of a primary challenger and intense pressure on her donors. DiFi's supporters are voting for her for a very simple reason: they (rightly) associate republicans with being anti-choice, anti-LGBT, anti-immigrant, pro-corporate, and unilateral when it comes in regards to foreign policy. They understand DiFi is horrible in regards to civil liberties, but they see any republican challenger as standing for something worse (and this is very much the republicans' fault).

tl;dr They aren't voting for DiFi based her civil horrible liberties (constitutional issues, WoD, NSA spying, etc...) stances: they're voting against republicans based on that party (vs. the individual candidate’s) based on the republican party's civil rights stances (women's rights, protection against discrimination, immigration, gay rights, etc...).

If, on the other hand, the choice is between two "liberal" democrats, the electorate will not feel that they're risking much.

I think steps are:

1) Find anyone with strong civil libertarian and general liberal credentials: one approach may be to convince liberal-leaning republicans with great degree of California popularity -- e.g., Richard Riordan or Tom Campbell -- to switch over, the other may be convincing a house representative to do the same (other approaches could be prominent attorneys, business leaders, etc...)

2) Exert grass-roots pressure on corporate and wealthy donors to support that candidate. Seek endorsements from national political figures. One (controversial) approach may be to have senators Manchin or Toomey speak in a campaign slot about how Feinstein's early push for an assault weapons ban sunk any chances of Machin Toomey passing ("Feinstein put the horse before the cart, rather than helping us get our foot in the door, she alienated moderate gun owners from the cause of gun safety" -- which is literally what happened).

3) At the mean time, promote a popular, liberal republican in the republican primaries as well. Likewise, sponsor a third party liberal (e.g., green) challenger. In other words, democratic donors and national democratic party leadership should begin to think that there's a likelihood of Feinstein losing, and that a candidate with more fresh appeal would be required to win.

4) Increase voter registration in the 18-29 group: they strongly oppose NSA spying, drug war, and support gun ownership more strongly than any other age group (in all of these issues, Feinstein stands diametrically opposed to them). They also vote the least, especially in primaries (many many not be aware such exist).

I am not saying this will be easy or that this strategy will succeed in 2016. Most likely, such a campaign will still fail, but not without giving Feinstein and the democratic establishment a great scare. It will certainly have an impact in 2022: the coalition will come back together in that year's democratic primary and either field an candidate that will be elected or get the establishment candidate to sway in a more civil libertarian direction.

tl;dr Stir up the existing consensus, form a new coalition around a candidate that goes against the Chumer/Feinstein wing of democratic party: the wing that was elected on a "tough on crime" platform as a result of political conditions -- consensus view between dems and republicans on economic issues, high crime rates, cold war winding down, etc... -- that no longer exist.

Republican party did this: Barry Goldwater overwhelming lost the election, but the coalition of pundits, donors, politicians, and special interest groups that formed around him helped Reagan get elected.