It's a bit weird that this is even a thing given Google's ecosystem supports retention and litigation holds - technology acquired through buying Postini iirc.
1) Use Postini
2) Violate an explicit court order by automatically deleting all your incriminating documents.
3) Blame thousands of unaware individual contributors for not each manually blocking << 1% of the illegal deletions, per directions that went straight to their corporate mailing list filter’s trash can.
4) Cite this case as precedent, and demand whatever slap on the wrist Google is about to receive.
1.5) Buy insurance to pay whatever that fine works out to.
5) Continue breaking the law.
6) GOTO 2 when the next subpoena arrives.