[pasbesoin]

Your only defense is compartmentalization. Segregate the data and encrypt each segment separately. Communicated the data and keys through separate channels to separate parties. Hope that, therefore, a compromise is limited to a single compartment.

You could also make decryption dependent upon a network connection (e.g. Adobe DRM, et al.), but with "the opposition" potentially in control of the network and/or able to compromise you physical security, and with the decrypted results readily copy-able (they always are, one way or another), this is probably more trouble than it's worth.

P.S. I didn't mean actually Adobe DRM; rather, just citing them as an example instance of such a thing (though, truth be told, I've never looked at how they do theirs, in detail).